Facial Expressions

Why?

Why do we make them?

More importantly, why do we make them in private? Like smiles or frowns.

We could very well be happy or sad or upset without smiling or frowning. But we do it. Most of us do it.

If I stub my toe in the shower, there is no need for me to frown, because a frown is just a communication to others that I am unhappy with something. In the shower, I am alone (supposedly), so why do I need to communicate my unhappiness?

I was just curious about this. Perhaps it's just practice for future person-to-person communication. Perhaps it's just habit.