Why are guys so quick to notice me glancing at them?

90% of the time when I turn around to glance at a guy deliberately to check them out, they always notice it immediately and our eyes would meet. Then I get shy and intimidated so I would immediately turn back around and act like I wasn't checking them out. This has happened a couple of times already. It's like they are hyper aware of girls looking at them. I can't even check them out in peace.

Why are guys so quick to notice me glancing at them?
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