Many patients find themselves getting very emotional when in pain.   Crying spells, depression, and moodiness are not uncommon.  This seems to be due to the fact that chronic pain is carried by slow c-delta nerve fibers which make their way along the spino-thalamic tract to the brain.  This system is old from an evolutionary stand point, so these signals end up in primitive parts of the brain responsible for controlling emotions.  A good example is an effect seen by massage therapists and body workers for centuries.  When working deep in a patient's muscles, many therapists note that the patient will sob uncontrollably.  The reason is that these pain impulses have a direct link into the parts of the brain that control emotion.

Another cause of emotional issues in patients with chronic pain is depression.