Why is it a girl is more chatty with me when the 2 of us are alone?

I've noticed if I'm talking to another lady she comes over and stands with us but doesn't stay long or say anything. She then wonders off and doesn't talk to anyone. But then she makes more effort to talk to me when I'm alone. She and I had a really good conversation when it was just the 2 of us.

Why is it a girl is more chatty with me when the 2 of us are alone?
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