Some time ago, I came across the All Souls College philosophy fellowship exam. It's interesting reading throughout, but one question in particular brought me up short when I read it.
What, if anything, is bad about pain?
The fact that I couldn't answer this immediately was fairly disturbing. Approaching it from the opposite angle was much simpler. It is in fact trivially easy to say what is good about pain. To do so, all you need to do is look at the people who are born without the ability to feel it: CIPA patients. You wouldn't want your kid saddled with this condition, unless for some reason you'd find it welcome for the child to die (painlessly) before the age of three, and if that fate were escaped, to spend a lifetime massively inconvenienced, disabled, and endangered by undetected and untreated injuries and illnesses great and small.
But... what, if anything, is bad about pain?
I don't enjoy it, to be sure, but I also don't enjoy soda or warm weather or chess or the sound of vacuum cleaners, and it seems that it would be a different thing entirely to claim that these things are bad. Most people don't enjoy pain, but most people also don't enjoy lutefisk or rock climbing or musical theater or having sex with a member of the same sex, and it seems like a different claim to hold that lutefisk and rock climbing and musical theater and gay sex are bad. And it's just not the case that all people don't enjoy pain, so that's an immediate dead end.
So... what, if anything, is bad about pain?
Let's go back to the CIPA patients. I suggested that they indicate what's good about pain by showing us what happens to people without any: failure to detect and respond to injury and illness leads to exacerbation of their effects, up to and including untimely death. What's bad about those things? If we're doubting the badness of pain, we may as well doubt the badness of other stuff we don't like and try to avoid, like death. With death, there are some readier answers: you could call it a tragic loss of a just-plain-inherently-valuable individual, but if you don't like that answer (and many people don't seem to), you can point to the grief of the loved ones (conveniently ignoring that not everybody has loved ones) which is... um... pain. Whoops. Well, you could try making it about the end of the productive contribution to society, on the assumption that the dead person did something useful (and conveniently ignore why we tend not to be huge fans of death even when it happens to unproductive persons). Maybe we've just lost an anesthesiologist, who, um.... relieves pain.
And... what, if anything, is bad about pain?
Your standard-issue utilitarianism is, among other things, "hedonic". That means it includes among its tenets hedonism, which is the idea that pleasure is good and pain is bad, end of story. Lots of pleasure is better than a little and lots of pain is worse than a little and you can give these things units and do arithmetic to them to figure out how good or bad something is and then wag your finger or supply accolades to whoever is responsible for that thing. Since hedonists are just as entitled as anyone to their primitive notions, that's fine, but it's not much help to our question. "It is a primitive notion of my theory" is the adult equivalent of "it just is, that's all, your question is stupid!" (I don't claim that this is never an appropriate answer. Some questions are pretty stupid. But I don't think that one of them is...)
...what, if anything, is bad about pain?
This is a form of the general question "What's so bad about X?" with pain as X.
For any X, we can ask "What's so bad about X", receive an answer X2, and ask "What's so bad about X2", ad infinitum. The three most common responses are semantic stopsigns, moral nihilism, and admitting you need to ask the question more rigorously.
Once phrased more rigorously, the problem becomes easier, transforming into some combination of:
"Why do people dislike pain?", to which the answer is that it's hard-wired into the brain in some way a neurologist could probably explain, probably similar to how it's hard-wired to dislike things that taste bitter.
"Why do people call pain bad?", to which the answer is that most people think as emotivists, and call pain bad because they dislike it.
"Why is pain bad in Moral System Y?", to which the answer is that you'd have to ask the people in moral system Y, and they'll give you their moral system's answer. I think a lot of the better moral system would have it as an axiom. They probably make it an axiom because most moral systems are linked in some way or another to what people do or don't like, whether they admit it or not.
"Why is there a strong negative qualia for pain instead of it just feeling like a little voice at the back of your head saying 'that's painful'?", to which the answer will remain mysterious until we understand qualia, but no more mysterious than any other sensation.
Excellent response.
As a side note, I do suspect that there's a big functional difference between an entity that feels a small voice in the back of the head and an entity that feels pain like we do.