Ever see someone "talking to themselves"? They're actually talking to an imaginary friend (or foe or relative), unless they're praying, of course. If we imagine the face of someone we know (or knew) and say something we would like them to hear, we're talking to an imaginary person, since that person is not actually there to hear what we say.
Here's an example: when I was a child I used to hear my mother talking to herself. I would ask her who she was talking to, and she would say "anyone who'll listen" - but even as a child I knew that was just an excuse.
We might inherently know that if we say things out loud to no one in particular (and not with a bluetooth), that people think we're off our rocker, and this is the reason why we don't do it in public. It's embarrassing that people notice if we speak to an imaginary person, because it tells of immaturity or addiction.
And I also wonder if we are embarrassed to verbally pray in public, because we actually believe that prayer to God is talking to an imaginary person? Certainly the atheist believes that God is imaginary. I just wonder how strong our faith is if verbal prayer is embarrassing.
TD