Is Love Natural?

With Valentine's Day (or VD, as it is sometimes abbreviated) coming up this weekend, let's think about love. Is romantic something we are biologically pre-programed to feel or is it culturally constructed, something we've been taught to feel? I'm not talking about crushes or the "oooooh" feeling one gets, but the "happily ever after" kind of love. Is it part of being human or part of being socialized in a culture like ours?