Why do we love?
There are evolutionary reasons for feelings we commonly associate with love. Love is what drives us to continue the survival of the species--rather than living briefly in the brutality of the State of Nature, we live in comfort through mutual sacrifice. We're compelled to form social associations for our collective benefit, and to produce and care for our offspring, and there are clear evolutionary advantages to doing so.
But at some point in the history of humanity, mere survival wasn't enough, and the impulses that governed our survival were turned to less vulgar targets and became ethics and aesthetics.