Self-confidence. Some women look at their flat chest in the mirror and think they look too boyish, so they want a bigger rack to feel better about themselves. They just want to look more shapely in a figure-hugging dress or swimsuit. Or they're not happy in other ways - shape, firmness etc. Some have one bigger than the other. Some do it because they think it might help their job prospects if they're trying to get into glamour or porn. Some do it because they want more guys to look at them, I've known one or two girls that did it because they were fed up of losing out to their friends with bigger racks. Some do it because they think it will make their current partner happy, some are (sadly) even pressured into doing so by said current partner. Myriad of reasons.