Drug use changes your brain chemistry
People initially use alcohol and drugs while engaging in other addictive behaviors to feel "good" or to self medicate the physical and/or psychological pain that may exist.  Alcohol, drugs and addictive behaviors directly stimulate and increase the brain's production and use of REWARD chemicals such as dopamine.  Depending on the dose and method of use, users may expose their brains to levels of neurotransmitters that are many times higher than they would normally experience during "natural" highs. This, in combination with the fact that drugs override the brain's normal mechanisms for self regulation, helps explain why drug addiction occurs.

How Dependence Changes the Brain
Continued drug abuse alters the way your brain works. Over time, the brain becomes increasingly dependent on receiving an external supply of the drug. These changes in brain chemistry and function play an important role in the physical and behavioral symptoms of substance dependence: tolerance, withdrawal symptoms, craving, and relapse.