Brief Answer:
There are two broad categories.

Detailed Answer:
Frankly the most common I see is not actually chronic pain even though everyone says it is: it is chronic-doing-bad-stuff-to-oneself. So, somebody has an injury but keeps doing the job/motion/activity that caused the pain. Even if the pain would damp down (and nearly always it would), they keep re injuring themselves and the pain won't go away. The main thing to do is obvious and yet doesn't happen.

Then there are situations in which the pain is due to a situation which is messed up:
Typical is a nerve injury and the nerve is made to think there is a constant rein jury but the problem is just in the nerve. Diabetes, viral infection of a nerve, cut nerves are some examples. Medications that just damp down the nerve are both highly effective and very very safe.

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