self talk

If there is constantly a whole conversation going on in our heads, why are so many of walking around like a robot. As if nothing is happening up there. When I am talking to a friend I am using my whole self. My facial expression, my body language and my voice are as much a part of a conversation as content itself. So why don't we do it with the conversation we have with ourselves. If you talk to yourself on the street a lot of people will give you weird looks and feel uncomfortable. As if you are insane. Paradoxically "talking to oneself" is actually very sane. We all do it, everywhere, all the time. You are just not supposed to show. Why?