is conjugated in the Usted form because it isn't you doing something, it's the thing doing it. So when you say "me gustas tu" what you are really saying is "You please me." Which is why it is conjugated in the tu form, because they are doing the pleasing. Te quiero, is more personal, sometimes romantic. It's literally saying "I want you," but it has kinda a different connotation, like "I yearn for you" or "You complete me" or kinda "you are my person." Depending on if you said it to a lover or a really good friend.