From what I understood human love lasts four years at most. It's possible to fall in love with the same partner again, but this seems to suggest that the human isn't naturally monogamous. Many animals stay together for the raising of the child and part ways afterwards.
Now, why does a part of humanity force monogamy on themselves? Hm, why do we enforce rules like "no porn or nudity for children" on ourselves (after all, the child has seen bare breasts shortly after its birth)? Why do we have other rituals like weddings, funerals, national anthems or pledges? There's no real reason for all that, humans just love to exact power by forcing others to behave in a certain way. The Alpha instinct. If your partner is tied to you for life you're exacting some form of power on him which satisfies your Alpha instinct. People buy "cool" things to show their superiority, people beat highscores and win games to show superiority and people live with honor show their superiority. It doesn't matter whether you consciously try to be better than others, you just feel good by doing so.
(I think one of the first people to push this theory was Nietzsche ("Der Wille zur Macht", before that book he assumed the basic instinct was procreation, however, those two seem to be pretty much identical. Power gives you control of partners and better genetic material). Previous philosophers assumed the human is driven by his intelligence and a rational being. Nietzsche tried to prove this basic assumption wrong, which would invalidate any and all philosophy that came before him.)