It is the case in heroin addiction that, initially, they take the drug for the high, but ultimately they take the drug to avoid being sick. The same is true, to some degree, in crack cocaine addiction. That is, we find that people coming off crack cocaine get depressed very badly, and so they are, in effect, medicating themselves, giving themselves crack cocaine to avoid the low. What they're trying to do is pump their dopamine levels up, which doesn't happen, but they keep trying to do it. Perhaps we should assure people that a certain level of dopamine is normally produced in the brain by pleasurable foods, or activities, and is necessary for human life.
Dopamine is a very important substance in many different ways.
It is, for example, involved in motor function. In order to maintain motor function, you must have a minimal amount of dopamine. Parkinson's disease is a deficit in dopamine levels, which results in motor problems. Both schizophrenia and depression have dopamine components to them, mostly schizophrenia. In fact, anti-psychotic drugs work on dopamine levels.