Cocaine and meth are both stimulants, but they affect the dopamine system in different ways.

 Cocaine prevents the reuptake of dopamine, while amphetamines cause more dopamine to be secreted.  The end result is similar — too much dopamine (and neurotransmitter by-products of dopamine) bouncing around the brain.  This leads to excitement and feelings of anticipation in the short-term, but severe dips in energy and motivation later on. The brain becomes less sensitive to dopamine as a way to protect itself and restore balance.  When the chemical stimulus is removed, the brain is still desensitized to dopamine and needs more to feel “normal.”
Behavior can densensitize even a drug-free brain.  Risky behaviors, pastimes, and careers (e.g. extreme sports, high-stakes finance, or professional gambling) can model the brain in powerful ways.  So can intense, life-changing experiences.  A brain that has been to war is not the same as a brain that has not been to war. Some people are attracted to novelty and excitement because of genetic factors that affect brain structure and neurophysiology, but behavior models our brains above and beyond the effects of genetics.

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