Smiling just happens. And people do it differently. like some people smile lots, others don't. I don't smile or laugh much. Partly because I'm a miserable fidget, but because my face, too, is not that enthusiastic. My smiles are little and my laughs quietish. And my bunnies numerous.

So some obnoxious people laugh painfully loudly and stretch up their stupid faces grinning like twits.
But no one's insecure for it. If you're really hiding sadness behind that fleeting smile, blame the other guy for not noticing. It's his lack of social skills.