Why do men make it so obvious when they stare at a woman's body?

I cover myself up. I don't wear sexy clothes bc I have a big chest size.

But there are always men who just stare for too long, sometimes with eyes and mouth wide open.
Even though I look at them, like; "excuse me?" And then I turn around.
My friend noticed it too but that friend thinks it is also bc I am good looking which I don't.
Why do men make it so obvious when they stare at a woman's body?
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