Why would a guy become tense around a girl?

Anonymous
This guy has friends that are girls and seems to be relaxed and himself around other girls but with me it is the opposite. When we met the first couple of times, he was more outgoing than I was, and super nice to me. I tend to be shy first hand but open up later on, however i'm still smiley and nice. Then suddenly we when met up again recently, he got quiet, shy and nervous... only around me though. Why is that?
Why would a guy become tense around a girl?
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