I used to sometimes. People expect you to react a certain way to certain things.
For example, disappointment ("oh, you failed at whatever, you should be so disappointed"), happiness ("you did so well in that exam, you should be so happy"), sadness ("oh isn't it said that random guy you don't know has cancer), arousal (some drunk slut hitting on you, most guys would expet you to be horny, regardless of anything else).
If you don't feel these ways that society expects, you're seen as weird, or strange, or they may be offended. Not any more though.
Yea, I do this just to appear normal and not draw attention. I haven't genuinely smiled or laughed in a long time. I usually just mimic the facial expressions and emotions, of who I'm with at the time. Anger is the toughest. It's just a social skill that I picked up.