I think that love is a duty. A duty to make the person we love happy. It's about overcoming our egoism, our flaws so that they can have the best partner. It's about making hard decisons to swallow our pride, to put our best interest on the second place. Few people can accomplish that so a real love is very rare. Most people leave their partners after they no longer need them. Many people don't even build a psychological relationship, they just limit it to sex.

Also, "The art of loving" by Erich Fromm is a good book to read.