Why do people feel the need to smile?

People feel they need to smile for a number of reasons. A lot of it is just natural. Healthy people smile when they are happy. But there are other reasons. A lot of that is cultural too. Children are criticized when they don't smile in photos and are accused of ruining the pictures. Women are often expected to smile by their spouses, employers, and society at large. It is typical for women when they are walking to give a quick smile to someone they pass, particularly another woman. A lot of that is just being polite and doing so because that is perceived to be the right thing to do.

Plus smiling is not always benign or a good thing. Some people smile when bad things happen to those they dislike. Perhaps the person hurt them personally or represents someone who once did. Depending on the level of hurt, they may even laugh uncontrollably. Sometimes smiling is just another way of expressing control and dominance over others. Bullies and serial criminals often smile too, and it is just one more way they torment their victims.