WHY DO GUYS ALWAYS STARE AT BOOBS?

I have pretty noticeable breasts. I dislike having them, feel akward showing my body off, or the attention. I don't feel comfortable doing that. I always wear simple clothes, shirts, jeans and shoes. Know that it's summer I wear a tank top, jeans, and very thin sweaters. I hate how guys always look at me and my boobs are like the first or second thing they notice! They also check out what I'm wearing, shoes, clothes etc. Annoying. I wonder how it would be if I wore less clothing, jeez. What can I do, I hate attention? Also, why are breast such a big DEAL FOR GUYS?