Caran: I think it's only a minority of people who stand facing a mirror whilst brushing their teeth. Most of the time it's because the tooth brush and tooth paste is kept in the bathroom or en-suite which both usually have mirrors above the sink. You have to look forward so you are bound to look at yourself. Simples.
Good point, Caran. Although, I suppose the mirror is there anyway, so we might as well check that we're doing it well enough. I worry about red wine stains etc.