In the cafe, a boy was staring at me constantly, I looked at him and looked away, but still he was staring, Why do guys constantly stare at a girl?

He was an acquaintance of my classmate. But at that time my classmate was not with me. I was with some other friends in the cafe. I never spoke to that boy and never met him. I found his behavior very strange.
In the cafe, a boy was staring at me constantly, I looked at him and looked away, but still he was staring, Why do guys constantly stare at a girl?
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