We seem to be the only primates with a pair-bonding urge. It's been argued that this is due to the greatly extended dependency of human infants: that doesn't stand up well when you consider that we also show signs of being adapted to co-operative child rearing (such as the ability of human females to lactate without pregnancy if they nurse someone else's baby for long enough,) but it might explain why a pairing that feels eternal when it starts can fade away after a few years, when an infant might be big enough not to need a couple's care.
But if it's a real instinct, it must have been overlaid on the much older primate promiscuity. And as so often happens when you add a software patch over an existing program, and launch it without enough debugging, the two sometimes interact in bizarre ways and sometimes crash completely.