Have you ever found yourself in the foetal position on the sofa for days on end, curtains drawn, phone unanswered, moving only to haphazardly wipe the snot and tears from your face? All that has happened is you’ve been made aware that you won’t be seeing a person you had a lot of interaction with much any more. That’s it. So why does it leave you reeling for weeks, months, even for the rest of your life, in some cases?

Humans seem primed to seek out and form monogamous romantic relationships, and this is reflected in a number of weird things the brain does when we end up falling for someone. Attraction is governed by many factors. Many species develop secondary sex characteristics, which are features that occur during sexual maturity but that aren’t directly involved in the reproductive process; for instance, a moose’s antlers or a peacock’s tail. They are impressive and show how fit and healthy the individual creature is, but they don’t do much beyond that.

Humans are no different. As adults, we develop many features that are apparently largely for physically attracting others: the deep voice, enlarged frames and facial hair of men, or the protruding breasts and pronounced curves of women. None of these things are “essential”, but in the distant past, some of our ancestors decided that is what they wanted in a partner, and evolution took over from there. But then we end up with something of a chicken-and-egg scenario with regards to the brain, in that the human brain inherently finds certain features attractive because it has evolved to do so. Which came first, the attraction or the primitive brain’s recognition of it? Hard to say.

It is important, however, to differentiate between a desire for sex, AKA lust, and the deeper, more personal attraction and bonding we associate with romance and love, things more often sought and found with long-term relationships. People can (and frequently do) enjoy purely physical sexual interactions with others that they have no real “fondness” for apart from an appreciation for their appearance, and even that is not essential. Sex is a tricky thing to pin down with the brain, as it underlies much of our adult thinking and behaviour.

But this isn’t really about lust; we’re talking more about love, in the romantic sense, for one specific individual. There is a lot of evidence to suggest the brain processes love differently. Studies by Bartels and Zeki suggest that when individuals who describe themselves as in love are shown images of their romantic partners, there is raised activity (not seen in lust or more platonic relationships) in a network of brain regions including the medial insula, anterior cingulate cortex, caudate nucleus and putamen.

There is also lower activity in the posterior cingulate gyrus and in the amygdala. The posterior cingulate gyrus is often associated with painful emotion perception, so it makes sense that your loved one’s presence would shut this down a bit. The amygdala processes emotions and memory, but often for negative things such as fear and anger, so again, it makes sense that it’s not so active now. People in committed relationships can often seem more relaxed and less bothered about day-to-day annoyances, regularly coming across as “smug” to the independent observer.

One type of chemical often associated with attraction are pheromones, specific substances given off in sweat that other individuals detect and that alter their behaviour. While human pheromones are regularly referred to (you can seemingly buy sprays laced with them if you’re looking to increase your sexual appeal), there is currently no definitive evidence that humans have specific pheromones that influence attraction and arousal. The brain may often be an idiot, but it is not so easily manipulated.

However, being in love seems to elevate dopamine activity in the reward pathway, meaning we experience pleasure in our partner’s presence, almost like a drug. And oxytocin is often referred to as the “love hormone” or similar, which is a ridiculous oversimplification of a complex substance, but it does seem to be elevated in people in relationships, and it has been linked to feelings of trust and connection in humans.

The flexibility of the brain means that, in response to all this deep and intense stuff, it adapts to expect it. And then it ends. Consider everything the brain invests in sustaining a relationship, all the changes it undergoes, all the value it places on being in one. If you remove all this in one fell swoop, the brain is going to be seriously negatively affected. All the positive sensations it has grown to expect suddenly cease, which is incredibly distressing for an organ that doesn’t deal with uncertainty and ambiguity well at all. Studies have shown that a relationship breakup activates the same brain regions that process physical pain.

Addiction and withdrawal can be very disruptive and damaging to the brain, and a not dissimilar process is happening here. This isn’t to say the brain doesn’t have the ability to deal with a breakup. It can put everything back together eventually, even if it’s a slow process. Some experiments showed that specifically focusing on the positive outcomes of a breakup can cause more rapid recovery and growth. And, just sometimes, science and cliches match up, and things really do get better with time.