Why would a guy look you in the eyes while talking to you alone, but if others are around he will not even look my direction at all?

We work together and he very obviously avoids looking at me when other coworker's are around. When it’s just us though he maintains good eye contact and even stares in my eyes at times.

Why would a guy look you in the eyes while talking to you alone, but if others are around he will not even look my direction at all?
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