Why would an outgoing guy go quiet around you?

So I was at a party with a guy recently and we were catching one to one and he seemed fine, then a friend joined in and he went really quiet. I like him but why would a guy be like that? He originally said hi and was going to walk past me, but I started the convo, then he seemed quite chatty one to one, just wonder why that would change when my friend came.

Why would an outgoing guy go quiet around you?
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