Guy's Behavior

Why would a guy stare at your eyes while you walk past him?

I was walking down the hallways (I spaced out for like 3-4seconds) then I look up you know the feeling when someone is staring at you... Then I see him staring at me... I looked at him back and his eyes where locked with mine but before they locked at the same moment I caught him his eyes flickered tried to quickly look away but it was too late because we where already locked staring at each other he kept on staring at me the whole time until I passed him but no smile. (He stares at me often, but no smile.) (also my neighbor)..
Why would a guy stare at your eyes while you walk past him?
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