What does it mean when guy has a serious stare?

Ok i know that "when guys stares at you, but doesn't approach, means he is shy but likes you. come talk to him..." blalala.
Im not asking about this!
I ask specifically about the resoan, of Why Would a guy stare at you But when he does it, he Always has serious/focused stare at you.
He just won't look away.
What does it mean when guy has a serious stare?
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