I also wanted to add in response to the OP's question of why Chronic pain (I'm not sure if this was said yet):
Given that pain is our body's way of saying there's something wrong, chronic pain means that something's always wrong, and that pain can be debilitating, it makes perfect sense that chronic pain exists: Those that are suffering have something wrong with them, and the pain reduces their chances of producing offspring, thereby promoting more fit generations in the future.
If an individual can reproduce before being debilitated by chronic pain, it doesn't matter since they were able to reproduce anyway, which is pretty much the key moment evolution is concerned with. This is also why a lot of people consult genetic counselors to get informed about the potential of spreading any of their genetic defects onto their offspring.
I know not all chronic pain is genetically related, but the point still remains valid: if the individual is more prone to pain, say, because of weak joints or some psychological disorder, their genes' chances of survival in the long run may be less than others if the vulnerability poses a significant enough problem.