In order to "go all teen hippy", you'd have to start out as something else, so I don't know why you bothered focusing on that.
http://lmgtfy.com/?q=secondary+sex+characteristic
Also, men have evolved over the years to prefer women with large breasts because it implies a greater ability to feed their offspring.
P.S. telling me I'm incorrect isn't the same thing as explaining why.
Edit: Neither is downthumbing.
You're incorrect, women with large breasts have the same marriage and pregnancy rates as small breasted women. British women have the largest overall breast sizes out of all the world and they have nowhere near the highest marriage or birth rates. Women with large breasts also have the same average amount of sexual partners as women with small breasts. For having something that men prefer, they sure aren't getting more men to fuck them.
Though it is just a theory, the Evolutionary Model suggests that mammary glands evolved to look that way to basically give males another butt to look at. The function of the glands to produce milk is the same as in all mammals but the shape and appearance of them is pretty much just a sexually selected preference. So as my previous comment states: Yay chest Butts!!!
http://lmgtfy.com/?q=secondary+sex+characteristic
Also, men have evolved over the years to prefer women with large breasts because it implies a greater ability to feed their offspring.
P.S. telling me I'm incorrect isn't the same thing as explaining why.
Edit: Neither is downthumbing.