Why does a guy stare at me from time to time?

this guy will always look in my direction and sometimes stares at me when I’m not looking

but whenever I look at him he looks away with kind of a mad face or like “why are u looking at me” type of face and he doesn’t look at me for along time and then stares at me again.
he also stares at me when I’m walking around but looks away when I look at him

why does he do this?
Why does a guy stare at me from time to time?
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