First of all, loobie, you clearly have the same virus that afflicts many other female ABers that makes them preface questions with "Why do you men..." ! Some of us DON'T, you know!
But anyway, that aside, you ask an interesting question. I think you're implying why do we like BIG ones? Personally, I don't. In fact, generally, I'm not much of a "breast man". I adore legs, I mean, really, REALLY love women's legs. But if you'd asked about THAT, I still couldn't really tell you why! If I do like breasts, it's small ones. So, you one or two here bemoaning your "small" boobs, be assured there are those of us out there who LOVE them. I generally like straight-down small-breasted boyish figures as well. Together with the fact that I love women having short hair, it makes me wonder if I have a teeny bit of gayness, but stop short of actually liking men and opting instead for women who are clearly feminine but a bit boyish!