I used to get that so much I came up with my own name for it. I called it Brampton Face. Brampton is the name of my sterile, soulless hometown. I considered my sad and pissed off default state to be a function of it.
I would get people telling me to smile and shit which really, misunderstanding or not ---> people like that should die.
I still get it a little bit - just the other day my friend's wife told me I looked serious when I wasn't at all - but whatever, it happens much less as I have gotten older and goofier in the head. I have become a surprisingly pleasant person. Yes, the resting face can still look sour but I spend a lot of time being pleasant and my face is pretty okay at those times.