My male colleague is nervous and shy around me?

My male colleague, always seems a bit nervous and awkward around me and I'm not sure why.

We don't always see each other but when we do, it takes him a while (hours!) to gain confidence around me. He sometimes appears to act quite hostile and just a bit awkward and quiet. Sometimes I'll ask him something and he'll look at me (almost like he's startled!) and he'll answer ever so quietly.

If we're ever on a conference call, it's the same - he doesn't joke with me the same as he does with the rest of the team. He'll have a laugh with everyone on the call but if I ask him a question or speak directly to him, he is very quiet and speaks slower and almost in a higher pitched voice.

Any explanations?
My male colleague is nervous and shy around me?
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