I swear, the more I smile, the grumpier I look. Maybe it's my innate shyness, but I just never take a good photo if I'm trying to look happy. Only occasionally am I caught in a spontaneous moment where I look like I'm enjoying myself.
It has something to do with having buck teeth as a kid, I think. I learned to keep my lips closed at all times, which gave me a tight smile. Although the mustache probably doesn't help, overall. I decided to shave it off for my fortieth birthday (Where now are the snows of yesteryear?), but got dissuaded by my Scouts -- and my wife. I don't think anybody would know me any more if I shaved it off now. (Maybe I'll do that when I retire.)
But even without the 'stache, I think I'd have to practice phony smiles in front of a mirror for hours to look anything like what I wish I did in portraits.