What does it mean when a guy stares but doesn't smile?

There is this guy in my community college that I notice staring at me whenever I'm near, but when I catch his eye, he doesn't smile, just stares very intently. My friends have noticed him doing that to me.

I've never met the guy and don't even know his name.

Does he think I am disgusting or something?
What does it mean when a guy stares but doesn't smile?
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