I have observed something odd that when you look into the mirror, the reflection changes the expression of your face. Some of the things I noticed were:
When a face has aspects that show a particular emotion, the reflection of the face has an expression of a person who thinks he is showing that emotion in his face but fails to do so.
Vice-versa, when a person thinks he is showing an emotion but does not do so, the reflection shows the exact emotion he is trying to portray.
I remember I used to think I couldn't smile genuinely, because the mirror image of my smile looked too insincere until I looked at a candid photo of mine. Does this happen to you too? I tested it by reversing photographs, to see the difference between mirror image and real life.