Why is he so weird around me?

I seen him around others and he seems more relaxed? I don't know how to put it.
I don't know if he’s just a two faced person. But when I’m in a quiet somber mood he stares at my face or eyes sometimes and it makes me think he’s studying me. Like he’s looking for or at something within me. It’s kinda weird.

when I’m happier he sometimes gives me the vibe he’s like scared or nervous talking with me. He’s outgoing so I don’t think he’s a shy guy realy. But he seems like he’s more cautious with his words or something I don't know. His face looks a little scared? Nervous? He didn’t used to really seem that way by the way.
this is lately. I’m confused.

He’s a coworker.

Why is he so weird around me?
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