In the UK, The National Health Service spent £442,000,000 on pain killers alone last year. In the USA the analgesics market is said to be worth $2,800,000,000, that's $2.8 billion, per annum. Clearly, pain is seen as a major medical problem and something to be overcome, even at considerable expense. Most people can imagine few things more distressing than constant chronic pain.
How many times have you heard piously self-righteous Christians and Muslims gleefully telling those who disagree with them that they can expect an eternity of pain and suffering for doing so? Suffering eternal pain seems to be the worst thing at least some humans can imagine.
There seems to be nothing to be said for pain at all. Yet we even have specialised nerve endings for feeling pain and centres in our brains for processing the information they provide and turning this into the conscious unpleasant experience we call pain. Indeed, the normal responses of our bodies to injury often seem designed to INCREASE the pain of injury.
So why have we evolved the ability to feel pain?
Put simply, pain tells us something is wrong. Pain draws our attention to injury or disease. Pain says do something or don't do something; guard me, rest me or don't use me. Don't walk on that broken ankle because it needs to be rested. Don't carry on with that chest pain but slow down and take a rest. Don't bite on that tooth or raise that broken arm. Close your eyes and sleep when that headache becomes unbearable. Put your hand over your ear when cold wind makes it ache and change your shoes when that blister bursts...
Pain even initiates reflexes which happen before our brains have noticed. These spinal reflexes have evolved to protect various parts of our bodies and pain is the signal to act automatically without the normal luxury of thinking about it first.
Pain has evolved as a signal. It is unpleasant because that tells us to try to stop it by resting or guarding the hurting part of our body. Pain is unpleasant because we have evolved to perceive it as unpleasant. Being unpleasant means we do something about it to reduce the unpleasant sensation.
Consider a patient dying in extreme pain of cancer, or an abscess, or a disabling injury in the absence of any pain relief? What possible purpose could that serve the individual?
Consider a gazelle dying of the shock of having it's intestines pulled out and its liver eaten by lions whilst still alive, or the zebra having a leg torn off by a crocodile as it is slowly drowned.
How does pain serve these individual?
Nature is unemotional and entirely lacking in compassion. Nature doesn't care about the suffering of a prey species as it is eaten and yet we can be quite sure that every sentient creature, and probably many others, feel pain. Nature has no concern at all for the discomfort or distress of an animal suffering from infection or dying of disease or simply starving to death of old age.
The fate of almost every living multi-cellular thing is to die of disease, or by being eaten, or of starvation due to injury or old age. There are very many ways to die and none of them are pleasant. Millions of feeling animals die every day in great pain. A system which has evolved to keep you alive is useless when you are dying, and yet it is still demanding you do something even when there is nothing you can do.
So why should we have evolved something we don't like and why would it be at its most insistent when at its most useless? What intelligent designer would design such a thing?
Because evolution isn't driven by what we like or dislike; evolution is driven by whatever ensures we have more descendants than we would otherwise have. Evolution is determined by what is in the interests of our genes because it is our genes which either survive in the next generation, or don't. And there is no benefit to our genes in evolving a mechanism to turn pain off when it is no longer any use.
So, evolution has provided us with something we don't like, and this is perfectly understandable in terms of mindless, unemotional, uncaring, genetic evolution.
What is not understandable is how this could have been designed by an intelligent, loving, caring and compassionate god. If pain has been designed by a god then that god must be a stupid, cruel, sadistic and hateful god.