I think we should also not forget the importance of movement in how we see ourselves. Smiling is a process. It has a beginning, a middle, and an end. In the mirror, you can see yourself making all the phases of your smile. In a photo, you only see one frame of it, and so the picture is flat and emotionless, even though you're "smiling".
How many pics have we all deleted, because most of us looked ridiculous in the single frame, even though we knew that particular moment was full of hilarity or joy? Meanwhile, the movie of that moment would have shown us all in a glorious light, living our best life.
We are not meant to have our lives captured in single frames, to be printed and placed in a book.
A related point: If you're looking in a mirror, you get instantaneous feedback on how you look—so if you don't like your look, you fix your hair, can change your expression, alter your head angle, and so on instantly until you get what you want.
In a photo, unless you're looking at the screen as you're taking it, the only feedback you get is after the fact.