The causes of some pain are unknown. In the past, these were explain as symptoms of psychological disorders. However, we know that chronic pain can lead to changes in the central nervous system. Sufferers develop a sort of pain memory, so that they continue to feel pain, even when the original trigger has long since disappeared. This helps to explain why some people develop hypersensitive reactions to harmless minor irritations.
The phantom pain that some people claim to feel in a part of the body that have been amputated may be result of a region of the brain recalling earlier pain signal. In the absence of these earlier signal, other signal produce same feeling of pain. Medication or electrical stimulation can help the brain to forget previously experienced pains.