why do men/boys look at women/girls butt?

Answer 1

It's partly genetics and partly learned behavior. Our genes tell us to look for attractive mates and the butt size is one of those attractors (big/small depends on the guy). Culturally guys have been taught to be attracted a ladies behind. So we've been trained to look at them when evaluating women. Not very gentlemanly I know. But you asked why.