I used to get that all the time, especially in middle and high school. Even now, I have to make a conscious effort to have varied/happy looking facial expressions. It's an odd concept to me to smile when I'm not actually happy or amused, but whatever it takes to keep people from constantly asking me that. It helps to make facial expressions in the mirror to make sure my muscles are moving correctly to give the right impression. Now I make a point to smile at people I know, and keep a little smile on instead of my "neutral" face. No one's been bugging me about it lately.
Also, supposedly it's an American thing to assume constant smile = normal/content. Makes me want to visit other countries where that isn't the case.
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