Girls, Why is it she can talk to other guys and be herself but with me she is quiet?

I've noticed she will make a small conversation when the 2 of us are alone. But she seems so quiet. But with other guys she can talk so easily and seems more confident.

But she doesn't have a problem with walking over and standing somewhere that puts her in my vicinity.

Girls, Why is it she can talk to other guys and be herself but with me she is quiet?
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