I agree with Hybridartifacts. Also, since many Americans have terrible teeth because they can't afford a dentist, the perfect smile has become a symbol of wealth and privilege. It's a way of saying 'breed with me; I have money and status'.

Tooth colour is partly genetic, partly environmental, partly dietary, partly age-related. As for crowding, which is the other main cosmetic issue, that's genetic. If you have a small dental arch, you're going to have problems.