Why do guys always stare at a womans nipples?

I am trying to understand why when my nipples poke out, guys get crazy over it.

When it is cold, my nipples get hard. That is just nature.

Why do I always have to try and hide my hard nipples when it is cold?

That happens to every woman.

If you could explain why women should have to (or not have to) hide their hard nipples I'd be interested in what you have to say about that...