Pain is controlled by stopping the brain from feeling it. Pain can be controlled at the level of the pain sensors, like with a lidocane patch or ice, at the level of the nerves that travel from the pain sensors to the brain, like at the dentist, and at the level of the brain. Some pain killers that affect the brain don't affect the brain's pain control centers. This is why Tylenol is a weak pain medicine with no street value.
Evolutionarily speaking, running and hunting with a wound or broken limb is superimportant to survival. Animals evolved a brain that creates its own opiods and has cells affected by opiods all over the nerves and brain. So while opiod drugs hit the pain control centers hard, they also hit many other brain areas, causing "improved mood and a feeling of well-being" as the medicos say.
When the system is flooded with morphine or oxycontin, it creates more opiod-sensitive cells over a matter of weeks. If the opiates are removed, the natural system doesn't work. Patients can feel even minor pain intensely, and suffer from low mood and a feeling of ill-being. Patients need to be tapered off to avoid withdrawal and the addiction associated with avoiding withdrawal.
It makes me very angry to see how use of effective pain control has swung from attention to addiction to attention to pain control and back. This is a stupidly oversimplistic approach to a complex problem.
