Chronic pain comes with a long list of possible causes, but they can generally be broken into 3 categories:
Chronic back pain, one of the most common forms of chronic pain, is a good example to use for this cause. Let's say that over time and through normal wear and tear, you have worn out the cartilage that surrounds the joints in your spine (the facet joints). The joints then become inflamed, and you develop osteoarthritis in the spine. That inflammation makes it very painful for you to move, and it's a source of constant pain. Your damaged facet joints and cartilage have now caused chronic pain.
The medical community isn't sure why acute (short-term) pain sometimes develops into long-term pain, even after the original pain source has been treated.
You can learn much more about chronic pain (including chronic back pain, as well as other types of chronic pain, such as fibromyalgia or rheumatoid arthritis) at our sister site Practical Pain Management.
Nerves can be injured in various ways. For example, a spinal nerve root can be pinched by a herniated disc, causing pain. Even after treating the herniated disc, the pain may persist because of nerve damage.
Nerves can be injured by arachnoiditis (inflammation of a tissue that protects the nerve roots), arthritis (again, the inflammation can compress the nerve), diabetes, cancer, Lyme disease, an infection, and more.
Nerves can have trouble relaying the appropriate message if they've been damaged. One theory about chronic pain is that the nerves don't stop sending pain signals to the brain, even after the source of pain no longer exists.
As you can tell from the previous two sections, there's a lot of uncertainty when it comes to chronic pain. Through research, the medical community is beginning to better understand pain and its causes, but all too frequently, they can't pinpoint a cause for chronic pain.
That doesn't mean that they can't help you find pain relief and ways to take control of your life again. It will be a process, but it is possible.