haha!

Ok, this butt theory sounds kinda weird but, hey, you never know!

Evolutionary biologists have discovered that a lot of the physical traits we consider attractive have a sound basis in genetics. Choosing the right mate obviously is a very important decision - so our genes have, over time, learned to help us out. We tend to find indications of health to be attractive. Signs of disease, malnutrition, or general unfitness are perceived as unattractive.
How does this explain the male fascination with big breasts? Well, it sounds like a reach, but here it is: humans are the only primates that have sex face-to-face. All other primates choose the front-to-back position. Human females are along among the primates in having large breasts even when not nursing. Evolutionary biologists theorize that large breasts look like a butt! Crazy as it sounds, all the evidence fits: all primates are turned on by butts - except people. Only humans have large breasts and mate face-to-face. Therefore, maybe big breasts evolved in imitation of buttocks.
Here's another explanation that's easier to believe: larger breasts indicate adequate stores of subcutaneous fat, which is a sign of health. Flat breasts may indicate a post-menopausal woman who is no longer able to have children and therefore not an ideal mate from the evolutionary perspective.