People with strong friendships survive 7.5 years longer than those with weak or few social ties, according to a Brigham Young University report that covered 148 previous studies, which included more than 300,000 participants. Friendship, it concluded, extends life.

Yet many friendships end. Why do so many friendships, even the long-term ones, go awry? Is it that people change? Their interests change? They relocate? Why do so many friendships break up, and most important, what can people do to mend friendships so they don’t terminate?

Don’t miss: Why loneliness in America is a public-health problem

When Marla Paul, author of “The Friendship Crisis: Finding, Making and Keeping Friends When You’re Not a Kid Anymore,” researched her book, she discovered that having long-term friends was “emotionally and physically protective. People have healthier cardiovascular systems; they’re less likely to die of a heart attack.”

In fact, she said, people with solid friends “sleep better.” Loneliness has a real physiological effect on one’s emotional and physical being, Paul suggested.

Keeping a friendship pulsating, fresh and dynamic requires a concerted effort and an investment of time. “You can’t take the friendship for granted. You have to show a friend you value him or her, make time for him or her, pay attention and be there when there’s a crisis to show emotional support via a dinner or a note,” Paul said.

“One thing people don’t understand is you don’t stop developing after you’re a teenager,” explained Tasha Howe, head of the psychology department at Humboldt State University in Arcata, Calif. and author of the textbook Marriages and Families in the 21st Century. People grow and change, and since many people live into their 80s, they could be friends with someone for 50+ years.

Facebook reveals big changes that will put friends ahead of business

The stages of friendship

Maintaining friendship, Howe noted, “is a lot to ask when you’re constantly changing and developing new interests and habits. Sometimes you realize that person isn’t just for the new version of you.”

Long-term friendships depend on “open communication, an equal power level, where you each influence each other and one person doesn’t dominate the other,” Howe said.

Paul acknowledged that friendships undergo their ups and downs, their difficult stages and satisfying ones. Therefore, “it takes the ability to let some feelings roll off of you” to sustain [the friendship],” Paul explained. If friends look at the big picture, in most cases, they’ll recognize that the positives outweigh the tough times. But if something is troubling you, you need to be able to talk it out with your friend.

Keeping the friendship alive and reinvigorated is critical. “You need to keep the friendship fresh, so it’s not same old, same old. Finding fresh adventures or discovering new things together” injects a dose of energy and variety into the friendship, Paul said.

Feeling valued is another key to keeping a long-term friendship alive and well. And that can be demonstrated and displayed in a variety of ways: a telephone call, sending a Valentine’s Day or birthday card to show you care, sending a text — anything to stay connected.

But friendships can lag and go through doldrums, so you may need to help them blossom. “If they add something to your life, if they make you grow as a person, then it’s worth continuing,” Howe said.

Is it time to call it quits?

Nonetheless, in some cases, long-term friendships reach a logical ending and terminate. Sometimes the friendship takes an unpredictable turn. If you’ve lived in the same neighborhood for years and the friend moves away, it can lead to an eventual dissolution of the relationship. Or if you’ve worked in the same office for years and one retires, it can disrupt the relationship.

Alternatively, if a friend “does something that feels like a betrayal, such as not showing up to your father’s funeral, or if [he or she] didn’t show up when you were in the hospital with cancer,” that friendship can take a nasty detour and end, observed Paul.

There are certain telltale signs that indicate the friendship is fading and it may be time to call it quits. When getting together feels like an obligation, you dread seeing someone or you walk away from dinner feeling angry, frustrated or dissatisfied, it may signal that the friendship is faltering and the end is near.

See: Why we have trouble kissing bad relationships and investments goodbye

Sometimes, friendships end because circumstances change. If you make a friend in cancer recovery and you’re healed but he or she is not, the balance of the friendship may change. Or if you are friends with a woman because your two married couples blend, but then you get divorced, the relationship may transform or you may feel like a third wheel. “Major life changes can affect a friendship,” Paul said.

For example, Lili Garfinkel, who lives in New York, had a lifelong friend whom she met in college. As their relationship evolved, Garfinkel started to become exasperated with her. Since her friend lives in Arizona, most of their interactions were by phone, but they included an annual visit. Garfinkel increasingly saw her friend as self-deluded, narcissistic and out of touch with herself. When she started confronting her friend about this, her friend became agitated and defensive.

Garfinkel reached the conclusion that “I no longer valued our relationship. That simply sharing a history does not legitimize continuing a relationship that was not worth my time.” Garfinkel valued her time too much to waste on a friend who was no longer simpatico.

Can the friendship be repaired?

When a friendship is fraught and conflicted, you have to decide “if you want to keep it going,” Paul said. Do you still value it? If so, you need to determine why it has lost its vigor, and whether there’s something to be done to reinvigorate the friendship.

Read: Will we ever know if money and relationships make people happy?

If your friendship has reached a difficult stage and could be ending, experts recommend these steps to determine whether to keep it going: