This comment in another thread:
CIPA. Pain is a very effective rapid educator. If you do something and it really hurts, you're very unlikely to forget and do it again. Pain tells us that something is wrong, and needs to be addressed. If your right leg hurts, you avoid putting weight on it.
But there's such an enormous number of people with chronic pain, and many of them to a very debilitating degree. I suspect this is likely the case with other animals as well. There's nothing that can be "fixed", so the pain ceases to provide any useful information. And the often-heard term "debilitating pain" indicates that not only does the pain not help anything, but in the most pragmatic sense it makes the creature less effective at doing anything useful.
It seems odd to me that chronic pain is so prevalent, and that natural selection hasn't bred it out of organisms in general. Like I mentioned above, it's debilitating, so it seems it would make an organism less fit for survival. Also, it's well known in the medical field that pain, especially strong and/or chronic pain, is detrimental to health. It's a major stressor, shortening lifespan and slowing healing (hospitals treat pain for the obvious humane reasons, but it's also been shown that people heal faster if their pain is managed). Wouldn't it stand to reason that organisms prone to chronic pain would tend to be bred out of a species, since they're less fit for survival? For that matter, I would think that pain would be a sexual deterrent, which would also make it less likely to be passed on as a genetic trait.
So why is it that nature tends to promote chronic pain, when it seems to be a great detriment with little or no benefit?