I am also miserable looking although I am a happy and generally cheerful person. I admit I sometimes get cross about politics and a few other things and when I do I probably look cross but it seems that the rest of the time, unless I am falling about laughing, I must look miserable.
Even as a child I'd be sitting quietly in the tram on my way to school to be told by an adult "Cheer up hen, it'll maybe no happen" I often got comments like that.
Perhaps it is because I am so miserable looking that I have always been a bit suspicious of folk who seem to smile all the time. I think it exudes insincerity and I tend to think of these far too smiley folk as putting on an act, although I must admit I have got used to Professor Brian Cox and think he is absolutely genuine. I did once hear him say in an interview that he got a lot of criticism for smiling too much, so I'm sure he just can't help it, just like I can't help looking miserable, even when I'm not.