The answer here is purely scientific.
Fact 1: Evolution and the principle of survival dictates that in order to survive, the a species must pass on its genetic information from one generation to the next. If this legacy is interrupted, that species eventually becomes extinct.
Fact 2: In order to ensure that the very best genetic information is passed on (and also to ensure that inferior genetic information is eradicated), individuals within the species develop certain traits to ensure that their genetic information will be passed on.
Fact 3: One of the traits that humans have evolved is the growing of larger breasts by females. This, scientifically, is supposed to show the males that the female would make a good candidate as a mother in nursing the males offspring.
That all being said... why do men find women with large breasts attractive? On a subconscious, evolutionary scale, its simply about our offspring. Nothing else. With this in mind, women are now able to enhance their breasts to compete with those women that already have large breasts. So its completely understandable why a woman would want larger breasts in the first place.
Science sure has a boring way of explaining non-boring things like sex, but it is what it is. Fortunately, it makes life really simple, if you follow it as a scientist. What is the purpose of life? Simply to propagate new life, and so on, and so on. Every action you make in your life is simply an effort to help pass on your genetic information, either directly, or indirectly.
Kinda makes you think, huh?