What Gnomatron said is generally true, but that's how our cultures and societies raise us. We are taught to look for a significant other of the same race, class, and religious background as ourselves, instead of looking at a person's personality. Ideally, couples will look alike because underneath it all, they have the same beliefs. Not necessarily religious beliefs, either, but a similiar outlook on life. Couples are supposed to be two parts of the same person, so it only makes sense that both halves of the pair would appear to look alike on some level, no matter what their race, gender, or physical appearance in general.