The shopping and spending activity itself is associated with a feeling of happiness and power, which is immediately gratifying. The after effects of remorse and guilt drive the spender back to purchase or shop again to be able to achieve that brief, but intense, emotional high.
Research has shown that many compulsive shoppers and spenders also suffer from mood disorders, substance abuse, or eating disorders. As with any addiction, the person becomes dependent on the behavior to relieve negative feelings that cause distress and discomfort.
They may experience anxiety or depression as a result of the spending or shopping, which may interfere with work or school performance.
See more at:
http://www.addictionrecov.org/Addictions/?AID=34