Why is he so uncomfortable around me?

He is a very talkative guy but around me, he is uncomfortable and not talkative at all! One time, he was giving me the answer and the entire time he was looking down. He stares at me, a lot and so does his group of friends. He never really tries to start a conversation with me as he had a chance to but did not take it. He talked to my friends, that he knows, when I around them but does not say anything to me. One time, he pass me in the hallway, and he looked surprised, mouth slightly open and raised eyebrows.
Why is he so uncomfortable around me?
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