Time after time I saw patients pleading with a nurse for more pain relief, and the answer came back, "You can only have so many in a 24 hour period, so you have to wait for another hour before I can give you any more."
The number of people I saw on one small ward who had hours each day full of really bad pain convinced me that pain relief would be one of the best places a philanthropist could spend money. If you're in that much pain you can't think properly, can't enjoy life, and it's all ultimately unnecessary, because, once a doctor and a patient agree there's something wrong and what to do about it, additional long-term suffering doesn't add anything to the experience.
People can have cancer. They can have arthritis. They can have a bad back, or kidney stones, or pancreatitis. I do think all those should be cured if possible. But the one constant misery for so many people, even if someone simply has old age - is pain.