Mating for life is part of human nature. The reason why it doesn't always happen on its own is that there are other parts of human nature too, and sometimes they contradict each other.

Remember, the fact that there are different "systems" in different species (pair bonding, harem-forming, random mating with whoever's there at the right time) means that there must be the possibility of intermediate states because evolution is gradual. Humans have a combination of traits that look like a cross between absolute pair bonding and the behavior of species that naturally form harems (the latter of which is a popular arrangement among mammals). The simplest explanation for this would be that we were forming harems in our evolutionary past and then started shifting over toward pair bonding, but haven't gotten all the way there at this time.