When you’re a natural smiler, it’s very hard to not smile. I tried. And failed miserably (no pun intended).
It’s easier to smile than to frown, because a smile receives more positive responses than a frown does. I used to believe it was a physical thing (i.e. “It takes 22 muscles to smile and 37 to frown”) until I googled it just now. What counts as a ‘smile’? Does happiness = smile? Or smile = happiness? We actually went through this question in Philosophy, asking whether we feel what we act (e.g. crying makes you feel sad) or act what we feel (e.g. feeling sad makes you cry). Personally I’m more inclined to believe in the latter, because I’ve smiled plenty of times without actually feeling happy at all, whereas all my genuine smiles come from feeling glad. Exceptions occur when we’re lying or in denial. Or in extreme cases, blackmailed.
As for what counts as a ‘smile’, I’ll let you think about that yourself. =)