People make distinct faces when they look at themselves in the mirror. It's particularly interesting to see the faces people make when seeing themselves in a store window. Sometimes people try to make themselves look extra photogenic with a Zoolander-like pouty face and other times people give themselves a smile or smirk.
It's curious that people make what they consider to be a private face in a public space. In a way, no one is bothered if strangers see it, but would be self-conscious if one of their friends were watching. To me, this is a reflection on the comfort of anonymity.
There's nothing wrong with doing this (of course I do it, too), but it's also something that we're typically uncomfortable doing with an audience. This privacy is an important part of us being able to express ourselves fully.