Hindsight is 20/20, as they say, and often it can lead to regret.

Some people have the philosophy that everything happens for a reason, and there is no point in having regrets.

But for so many of us, looking back on our lives can lead us to linger on one poignant moment or period when we wish we had done things differently, and that nagging question, "What if ...?," plays on repeat in our heads.

"This may sound a little melodramatic, but no matter how happy you are, at my age your regrets are countless," writes Quora user Gary Teal in response to the question, "When people look back on their lives, what are common regrets they have?" "You have made decades' worth of little miscalculations you can't completely erase from your memory, as well as a number of big mistakes that made life permanently harder."

What's your biggest regret?

Quora user Bradley Voytek points to a national survey about the regrets of a typical American, which found 13 common sources for regret. They are, in order: romance, family, education, career, finance, parenting, health, "other," friends, spirituality, community, leisure, and self.

Vaughn Bell at Mind Hacks notes that there are two ways people frame their regrets: The things they did that they wish they hadn't, and the things they wish they had done but didn't.

"The difference between the two is often a psychological one, because we can frame the same regret either way — as regret about an action: 'If only I had not dropped out of school;' or as a regret about an inaction: 'If only I had stayed in school.'

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"Despite the fact that they are practically equivalent, regrets framed as laments about actions were more common and more intense than regrets about inactions, although inaction regrets tended to be longer lasting," Bell writes.

Here are some of the most common regrets as chronicled by Quora users (answers have been edited for clarity):

Romance

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"I regret that I never fell in love with someone who was in love with me, when that would have been easy for me to do.

"I regret being like an old song sung by Buffy Sainte-Marie: 'Must I go bound and you so free, Must I love one who doesn't love me, Must I be born with so little art, As to love the one who would break my heart?'" —David Kahana

Children

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"For as long as I can remember, I have wanted to have kids. But in my younger years, I foolishly assumed that unlike certain accomplishments like a career, the marriage and kids thing would just happen.

"Well, they didn't. I dated plenty of people but never even thought about making family a priority. Then, in my late 30s, a bout with ovarian cancer left me permanently infertile.

"I think about the kids I never had every day, several times a day. I have a great relationship with my nieces and nephews, and volunteer at a children's hospital on a regular basis, but it's just not the same to be around other people's kids. I would love to adopt or be a foster mother, and hopefully be in a financial and domestic situation that would make this feasible one day.

"But again, not the same. And it pisses me off when people say, "You're lucky you don't have kids, they're so much work, blah blah blah." Yes, but a lot of things in life that are worthwhile are also so much work.

"I think the mothering instinct is so strong in some women that the knowledge that one will never get a chance to give birth and raise their own child goes beyond regret. One that a bar chart cannot capture. I can deal with most of my other regrets in life but am having a hard time dealing with this one." —Caroline Zelonka

Parents

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"I regret not choosing to spend more time with my parents in my twenties. I lost my mother in 2000, and I feel the loss of the friendship we never had.

"She was very demanding, very strict, and from the perspective of a young man, very unreasonable. It turned out, as I live through middle age, that most of the ideals I have today ended up being the ones she put on me.

"Sometimes, after a setback, I feel the impulse to call her, and in the second or so that it takes for me to realize she isn't alive to speak to any longer, I realize how much I still need her.

"You cannot negotiate with death. It is final, often sudden, and personal. The last night I had with her, at a hospice in Chicago, I was exhausted and asked her if she minded if I went home. She immediately whispered that absolutely, I should rest, and to be careful driving home. I curled her fingers around the nurses call button, and kissed her on the forehead. I remember I felt some relief that I was leaving.

"I know it didn't make a difference, leaving at that time, or leaving a few hours later. She was going to die either way. But reflecting on that moment today I know then that I didn't understand how precious those minutes were, and how a door was being closed that would never open again." —Jim Wagner

Education

Students at the University of East Anglia may have their caps digitally added into photos for an extra £8 (Getty)

"I wish I would've followed my dreams when I was in my late teens and deciding what I wanted to take in University. Had I made the decision to believe in myself — my talents, my passions, and my dreams, instead of worrying about getting a good job, my life would be very different today.

"I realize it's never too late to be true to yourself and honor your dreams, but I do wish I could talk to me 20+ years ago. If I could, I would tell my young self to research all of my options before diving into a (very expensive) post-secondary education that resulted in nothing more than mounting student loans and a place in the job hunting pack.

"I would've told myself that there is no such thing as job security and that I'd be better off putting the money I saved for post-secondary education into a small business where I would receive a real education while (most likely) making a very good living.

"Post-secondary education is a major investment, like buying a house. It shouldn't be rushed into, but pursued when you are ready and know what you want to do and what you want to get from it. Yes, if I could go back to my youth I would've listened to my soul and began writing screenplays. Who knows what kind of treasures I could've given the world if only I had listened to my heart instead of everyone else." —Stacey Grewal

Career

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"I regret not having had the courage to follow my calling. In my early 20s, I was too nervous to give performing arts a try as a profession, although I was very good at it. I love my current job, but if there is anything above it, it would the career I chose not to try.

"In my 20s I was also good academically and happened to see the lives of some professional artists, which scared me since I have always been risk averse, and so I chose to stay in the relatively secure confines of a career based on education and higher level degrees. It has taken me years of hard work to reach the secure position I am in currently, and I often wonder how it would have been different in the world of performing arts. If I had stayed and "paid the dues," would I not have reached the same level in that field doing my calling?" —Paul Klipp

Finance

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"If I could speak to my 25-year-old self, I would say: Don't worry about money so much. Take more chances. Take a year off, explore the world. It's not too late, but it gets harder, even without kids.

"Don't listen to that voice that says: If you quit this job or lose this income you'll never find another. You will." —Gabriel Friedman

Parenting

Those with limited social skills have a higher chance of getting arrested, binge drinking, and applying for public housing (Getty)

"I have but one regret: the time I did not spend with my children when they were young.

"I was the typical type A, work obsessed American male operating under the false belief that working hard to improve our financial security was my primary goal and responsibility.

"I was wrong.

"That time is lost, and while I cannot get it back, I do tell my children (and any others that ask when this topic comes up) that I am sorry I made that choice and to not repeat my mistake (and yes, I made a course correction once I got my priorities straight).

"Since I am very fortunate to have a kind, caring, and forgiving wife for the past 38+ years, our children turned out OK. But, when we look through pictures of those years, there is something noticeably absent ... me.

"Should you choose to have children, they need to be your priority. The sporting and school events, family outings, and simply time not spent with them can make a significant difference in your children — and you." —Tim O'Pry

Mental health

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"Much of the regrets that people accumulate are rooted in poor mental health. There are literally millions of people who live with undiagnosed low-level depression that emerges through their 20s and becomes increasingly severe and debilitating in their 30s. For me, this manifested itself in the form of self-medication with drugs and alcohol initially, followed by ill-fitting career choices. Over time, negativity comes to dominate the internal monologue and you begin to despise and short-change yourself. This can lead to many of the inactions that we later regret. Self-doubt is a spiral of inertia and paralysis.

"Some never recover, some do. For me, recovery began with the realization that something was seriously wrong. Mental illness carries a stigma that keeps us from being honest with ourselves about what's really going on. Seeking professional help is important, but you do most of the work on your own and it can't begin until you admit that you're unwell and choose to take action." —Michael Weston

Physical health

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"Health brings a freedom very few realize, until they no longer have it."

Friendship

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Lara also shares:

"I wish I had stayed in touch with my friends.

"Often (the dying) would not truly realize the full benefits of old friends until their dying weeks and it was not always possible to track them down. Many had become so caught up in their own lives that they had let golden friendships slip by over the years. There were many deep regrets about not giving friendships the time and effort that they deserved. Everyone misses their friends when they are dying."

Charity

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"The time when I chose not to stop and help a stranger who clearly needed it, justifying my decision with the fact that I was already running late and did not want to take the chance that I would be drawn into something time consuming and filled with drama. The brief look that passed between us has stayed with me." —Ann Cascarano

Loneliness

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"When I was young, I was both very cocky and very insecure. I thought I knew everything and was terrified that I might actually know nothing. So with too many things in my life, I tended to hack away at them in isolation, instead of asking for help from older, more experienced folks.

"Of course, not all advice is good advice, and not all mentors have only your best interests at heart. But that's OK. Asking for advice is like anything else in life: The more you do it, the better you'll get at it. You'll learn to ask the right questions and disregard advice that just doesn't feel appropriate to your circumstances. Eventually, you'll find a mentor — or if you're really lucky, several mentors — you can trust." —Andy Hermann

Travel

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"My only regret is not traveling before I was 24. Since then I've seen an enormous piece of this earth, but 24 was the most formative year of my life specifically because I travelled, and specifically it set the tone for the rest of my life as a traveller. Had I shifted that up I'd be 5 years ahead of where I am now in matters like confidence, experience, and wisdom, really." —Jesse James Richard

Worry

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"In our research at Cornell University, I asked hundreds of the oldest Americans that question. I had expected big-ticket items: an affair, a shady business deal, addictions — that kind of thing. I was therefore unprepared for the answer they often gave:

"I wish I hadn't spent so much of my life worrying.

"Over and over, as the 1,200 elders in our Cornell Legacy Project reflected on their lives, I heard versions of "I would have spent less time worrying" and "I regret that I worried so much about everything." Indeed, from the vantage point of late life, many people felt that if given a single "do-over" in life, they would like to have all the time back they spent fretting anxiously about the future.

"Their advice on this issue is devastatingly simple and direct: Worry is an enormous waste of your precious and limited lifetime. They suggested training yourself to reduce or eliminate worrying as the single most positive step you can make toward greater happiness.

"The elders' message is also consistent with research findings. The key characteristic of worry, according to scientists who study it, is that it takes place in the absence of actual stressors; that is, we worry when there is actually nothing concrete to worry about. This kind of worry ruminating about possible bad things that may happen to us or our loved ones — is entirely different from concrete problem solving.

"A critically important strategy for regret reduction, according to the elders we interviewed, is increasing the time spent on concrete problem solving and drastically eliminating time spent worrying. One activity enhances life, whereas down the road the other is deeply regretted as a waste of our all-too-short time on Earth." —Karl Pillemer

Self​

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"Don't try to please people.

"Nobody is more worthy of love in the entire universe than you. I wish I had reminded myself of that more.

"I could've saved all of that time where I was trying to please someone else.

"Money you lose you can always make back. But even five minutes of time lost is gone forever." —James Altucher