How come every time I talk to my best friend in public she looks around to see who else is listening?

Then she will say I talk about the wrong things at the wrong time no matter what topic it is.
i told her I don’t care what people think and most of them aren’t even listening at all anyway or paying attention and she looked at me shocked as if that statement is untrue. She’s the only friend that has said this to me though.
How come every time I talk to my best friend in public she looks around to see who else is listening?
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