Male coworker is friendly with other girls, but is hesitant with me?

Male coworker is friendly and outgoing. He is popular with other girls. He talks to them with ease, jokes around, but he remains distant with me.

However, I have noticed that he stares at me from a distance quite often and tends looks down quickly when I catch his gaze.

He also regularly locks eyes with me, but when I talk to him he seems to shut down and go reserved.

He found out the other week that I’m not single and since then doesn’t make eye contact at all and has become cold.

We have to work together at times and it feels very awkward.

Why can’t he engage on a friendly level when he does with other girls?

Male coworker is friendly with other girls, but is hesitant with me?
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