Is it bad to be serious?

I was at the Dentist today for a check-up (no cavities) and as I walked in, the receptionist said, "Why so angry?" The thing is, I wasn't, but I didn't really have a snappy come-back (social anxiety plus it was the dentist plus it was early in the morning). People, irl, generally seem to think I'm pissed off, I guess, because I don't perpetually smile. I usually have to explain, "that's the way I look" and then I feel like I'm defending myself, and not very well.

It's not that I'm unhappy, but I do almost always have things on my mind. I don't know... is smiling [u]all the time[/u] all its cracked up to be?