People who don't know me well tend to react as if I'm suffering terminal depression. This is probably because my face at rest can look sad, angry, put upon, annoyed, or irritated, depending upon the mood of the observer.
"You never seem to smile. What's wrong?"
Nothing. My neutral expression is sort of a Rorschach test and, yes, I don't smile that much and here's why, courtesy of Basic Instructions (I'm going to have this printed and laminated so I can hand it out to casual acquaintances):
Don't think I'm not thinking similar thoughts, by the way.