Why would a guy stare at you and watch you but act very cold or slightly unfriendly only with you?

He switches between being friendly (as he acts around everyone else) and being very cold with me as if we had a Cold War going on. He also watches me or steals glances at me whenever he can. What’s the deal?

Why would a guy stare at you and watch you but act very cold or slightly unfriendly only with you?
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