Guys, Guy staring deeply into a girl's eye?

There's this guy and he's lately been staring deeply into my eyes like when we bump into each other at work he will just start looking deeply into my eyes for what seems to be a long time. I look back and smile but he seems to be serious while I'm just like yep. What is he thinking and what does this mean?
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Guys, Guy staring deeply into a girl's eye?
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