My biggest pet peeve is when people impose on me to show happiness of excitement. There are subtle ways and not-so-subtle ways. No-so-subtly, people will ask me occasionally if I'm ok, why I'm not not smiling, why I'm not happy, why I look so sad, etc. Most of the time I'm actually just calm and reflecting on things which makes the inquiry not only imposing, but offensive, as if to say there's something wrong with not talking or not engaging. Fuck off. Even if I was sad, there are better ways to get a person to open up than to "accuse" them of violating social norms.
Then of course there are the less obvious ways people demand artificiality:
Validation requests. There people Some people will leave clues for you to validate them and show you how much you appreciate them. Even if you're not feeling it, their hints force you to seek or feign feelings that aren't necessarily there. These people lack a strong, stable self-image and need the validation of others to rid them of feelings of worthlessness.
The "yay!!!!" and other tools. Watch a group of people when one of them starts to yay!, and the rest will start yay!ing with them. Yay!ing is a way of forcing others to mirror one's level of fake enthusiasm and return it. A failure to respond with another yay is often taken as a social rejection. Yay!ing, of course, is only one way people force energy levels to go up. Another way is through constant, stentorian laughter, post-joke glaring that force the listener to laugh, or even blasting techno music all day. These folks can't tolerate quiet and calm environments and need to pump the atmosphere with "positivity," even if it's contrived.
It's interesting to see how people who are more dominant tend to reject these demands to approval or excitement, while submissives accede to these requests. I personally can't tolerate this (even when I make my own requests for validation), and think it's a great introvert-marker. It drains me like nothing else.