I have severe depression, but when I meet a joke or understand why people laugh, I will smile for fifteen seconds and then keep a straight face. Is this depression?

This may just be a state of mind. Sometimes the problem is not depression, because it is very serious for depression, and sometimes it may lead to clinical symptoms or clinical manifestations of other body functions, such as rapid heartbeat. So in this case, if not, it may just be caused by your own mentality or psychological reasons.

It is certainly not normal for such a situation. For you, there may be some problems in your heart, that is, there may be psychological obstacles. Therefore, the criteria for judging a psychological disorder are very strict. Sometimes we have to pass some psychological tests and simple questions, but also conduct a psychological analysis in reality to determine the final result. So here you can tell the general situation of your heart, especially some repressed things that you can't bear and can't solve, and then try to help you solve them.