why do guys only like girl with big butt??

I think its a primal instinct kind of thing for courtship. It's like why male deer have huge antlers, they aren't just to fight off other males but to impress females. Every animal of both genders have instinctive traits that can attract mates. Humans have these traits as well but we've become evolved enough to not let that influence who we couple with. However it is still buried deep in our genetics, a big butt usually is lead what would be called "child bearing hips". This can also go with why some guys like girls with big breast too. Personally I don't care too much about either, but I know some do cause thats how most (if not all) guys get labeled as jerks right off that bat.

Every person has stuff that they are attracted too, but if they are only into looks then their pretty shallow. Look for someone who brings out the better person in you not the person who wants to change you physically or mentally. That's the only advice I got.