Guy's Behavior

Why is he always staring at me with wide eyes? then when I look at him he stops?

There’s this guy at my work who is constantly staring at me when I’m doing something. For instance, there were 4 of us standing in a group having a chat. I was casually talking about something to the group, and when I looked over at him, he was staring at my face with wide eyes and almost immediately snapped out of it after I looked at him.
I know I’m attractive, I’m told all the time that I’m beautiful/hot/etc. and my personality is cool too.

I’m a petite and fit woman with a free spirit but I also stand up for myself and take no shit.
not sure if my personality intimidates him or something.
But I’m just wondering why this guy acts cool when I speak to him but when he thinks I’m not looking he just stares at me like a deer in headlights, almost a shocked expression.
Why is he always staring at me with wide eyes? then when I look at him he stops?
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