I’m quoting this from Brainpickings, who took the following passages from Jesse Bering’s book titled Why Is the Penis Shaped Like That?: And Other Reflections on Being Human.

[T]here are two main stages associated with a dead and dying romantic relationship, which is so often tied to one partner’s infidelities. During the ‘protest’ stage that occurs in the immediate aftermath of rejection, ‘abandoned lovers are generally dedicated to winning their sweetheart back. They obsessively dissect the relationship, trying to establish what went wrong; and they doggedly strategize about how to rekindle the romance. Disappointed lovers often make dramatic, humiliating, or even dangerous entrances into a beloved’s home or place of work, then storm out, only to return and plead anew. They visit mutual haunts and shared friends. They phone, e-mail, and write letters, pleading, accusing, and/or trying to seduce their abandoner.’

At the neurobiological level, the protest stage is characterized by unusually heightened, even frantic activity of dopamine and norepinephrine receptors in the brain, which has the effect of pronounced alertness similar to what is found in young animals abandoned by their mothers. This impassioned protest stage — if it proves unsuccessful in reestablishing the romantic relationship — slowly disintegrates into the second stage of heartbreak, what Fisher refers to as ‘resignation/despair,’ in which the rejected party gives up all hope of ever getting back together. ‘Drugged by sorrow,’ writes Fisher, ‘most cry, lie in bed, stare into space, drink too much, or hole up and watch TV.’ At the level of the brain, overtaxed dopamine-making cells begin sputtering out, causing lethargy and depression. And in the saddest cases, this depression is linked to heart attacks or strokes, so people can, quite literally, die of a broken heart. So we may not be ‘naturally monogamous’ as a species, but neither are we naturally polygamous.

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[O]ne of the more fascinating things about the resignation/despair stage is the possibility that it actually serves an adaptive function that may help to salvage the doomed relationship, especially for an empathetic species such as our own…. [H]eartbreak is not easily experienced at either end, and when your actions have produced such a sad and lamentable reaction in another person, when you watch someone you care about (but no longer feel any real long-term or sexual desire to be with) suffer in such ways, it can be difficult to fully extricate yourself from a withered romance. If I had to guess — in the absence of any studies that I’m aware of to support this claim — I’d say that a considerable amount of genes have replicated in our species solely because, with our damnable social cognitive abilities, we just don’t have the heart to break other people’s hearts.

Pretty interesting to read a scientific explanation for something that we usually assume is unexplainable.