Many things in the world want you to suffer. Signalling suffering is useful in many social situations. For example, suffering is a sign that one has little slack, and so entities that are out to get you will target those who signal suffering less.
Through Newcomblike self-deception, a person can come to believe that they are suffering. The easiest way to make yourself think that you are suffering is to actually suffer. In this way, the self-deception hyperstitions itself into reality. Perhaps a large amount of human suffering is caused by this.
Solving this problem may be of great interest to those who want to reduce human suffering.
I may write a longer post about this with more details and a more complete argument. If you particularly want this, please comment or dm, as that will make me more likely to write it.
Sometimes you want/need other people to help you, and if you display less suffering, they may assume that it's not serious, and therefore won't help you. This can be a problem for people who do not display suffering in neurotypical or culturally expected ways.
Sometimes there are situations where you are not allowed to say "no", and then "I can't, can't anymore!!!" becomes the next best thing. Or sometimes people just suck at saying "no".