I don't have this problem at all, it's just that I had a deformed jaw and had surgery and half my face is numb now. I smile all the time, but I look like I'm furious at someone. Sometimes when I'm getting my hair cut and look at myself for a long time, it's eerie and weird. I'm relaxed and happy, but I just look like i want to kill someone. I sometimes wonder if that's why people react the way they do to me.
But yeah, when I was growing up my family was always telling me to smile. I think that's also part of the problems of Autism - like so many other things, we're told to do as everyone else does, and we end up getting so confused between what we're doing, what we're supposed to be doing, what everyone else is doing, and trying to BE some thing we can't even identify - I think this screws us up more than our brain anomalies do. No wonder we're still confused way into adulthood.