[O]ne of the more fascinating things about the resignation/despair stage... is the possibility that it actually serves an adaptive signalling function that may help 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—and this is just a hunch, 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.The reason the sadness has to be legit is that humans are super savvy at detecting conscious deception ploys, and sadness recognized to be fake is not persuasive. So although one might prefer to merely fake sadness and otherwise go on as normal, that strategy has lower evolutionary fitness.
Bering's argument uses reasoning very similar to Michael Vassar's speculation that more social animals are more likely to feel pain. The only problem with both of these claims is that there's little direct data to back them up... can you think of any ethical way to test this?