Boy, if I had a nickel each time I've been told this... If the size difference between your two breasts is truly noticeable, then surgical adjustments might be needed. Usually, however, the size imbalance is not so big that it warrants doing anything about it.
Why does it happen? Well, take a look at yourself below those breasts for some answers. Your feet: Isn't one foot bigger than the other? Yes. Your hands: Does your wedding ring fit slightly looser or tighter on the other side? Right. And if you really want to do the full inventory, check out your ears too. Our bodies are designed that way. (Hmmm. I don't actually know if men's testicles are the same way, but I predict that they are.)
I think its kind of fun that our corresponding body parts are not exact twins. Why, even the DIEP flaps—which is the type of reconstructed breasts I have—are not exactly symmetrical. The doctor asked me at the time if I wanted him to do a little tweaking by liposuctioning the left one to make it slightly smaller, but I said, no thanks. My left one (original set) was slightly larger than the right, so I'm fine with this.
Our breasts go through many changes in our lifetime, too. They are destined to be bigger, smaller, higher, and, sadly, lower in the future. Puberty, pregnancy, childbirth, breast feeding, menopause, and aging will all make us care even less about one breast differing slightly from the other. So embrace yourself and the woman you are. Whether your bra cup sizes are a size B cup on the left and a B+ on the right, they are yours and reflect the uniqueness we all have as women.