Ah the age old question... It goes back to human development and the evolution of mankind. Boobs and butts are similar but back when we walked on all fours we had in front of us... the butt. As time passed and humans transitioned to walking on two legs we stopped having butts in our faces all the time and instead we were eye level with boobs. Women grew large breasts to take place of the buttocks. So therefore the original source of life at the beginning of human evolution was the buttocks. Boobs are just a cheap substitute, nothing more than a pale imitation of the buttocks. If you were asked if you would want a copy or an original, naturally you would choose the original.