Psychology: Why do human beings find butts attractive?
Men like to look at women with 'good' butts. The reasons for this are rooted in evolutionary psychology.
Primitive people were hunters and gatherers. The man would go out to kill animals so that he can bring food for his family. Chasing, trapping and killing an animal required the man to have an athletic built. The more athletic a man is, the more chances of him being successful in bringing food for the family. Women preferred to start their families with such man who can provide food security. They preferred men with broad shoulder, chiselled torso, muscular arms and legs (and butts!), and good height. Also these men have got good genes she can pass on to their offspring. These preferences got hardwired in their brains and got carried forward through generations. Women today are still attracted towards fit men with broad and muscular physique and a dominant personality.
In the same manner, women used to stay at home, give birth to children and take care of them. A good and round butt means a healthy reproductive system. During pregnancy, there is a drastic forward shift in the center of mass in women bodies, so a fuller butt and vertebral wedging counter acts to shift their center of mass back over their hips. Similarly, men used to believe that women with big breast would become good lactating mothers for their offspring. So they preferred women with good butts and breasts.
These preferences were passed through generations and good butts and breasts still make men lose their train of thoughts.
Men will be men!!