User blog comment:DYBAD/Love Without Selfishness/@comment-3075077-20130705143014/@comment-4867780-20130705165402

I think the key point about love is that, unlike common assumption, its purpose is not the happiness of the people involved. Love originally exists to create a strong bound of passion and attachment between a man and a woman, so that they have lots of sex, and so that the man stays at the woman's side to protect and nurture the offsprings.

And that's it. it is a merely reproductive and survivalist tool, nothing more. Its purpose fulfilled, love simply ends, and the two don't know what to do with each other anymore. Normally that wouldn't be a problem, since they should both be dead at this point (short lifespawn). But we live pretty damn long nowadays, so the partners invariably outlive their feelings for each other.