Chemistry? Yeah, chemistry.

It's no use trying to force chemistry. Either it's there, or it's not, or so they say. But the question remains, what is it that makes us attracted to one person and not another? There's scientific research about facial symmetry, and there's speculation about pheromones. There's the Freudian interpretation that we look for partners that resemble our opposite-sex parent physically and behaviorally, but that doesn't offer much explanation for people with an absent parent or same-sex attraction. And no amount of research can comfort you when you realize that it will never work with the person you're intellectually attracted to simply because you have no physical attraction to him.