Why does my crush acts so loud with his friends when im around, but when we talk, he's very quiet?

Me and my crush aren't at the same group of class but when we meet at hallways he's always very loud with his friends but when we talk alone he's very quiet, his tone of voice changes, and he smiles... I've tried to look for some info for why does he smiles at me... help?
Why does my crush acts so loud with his friends when im around, but when we talk, he's very quiet?
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