A normal pleasurable event begins with a stimulus. I see my young granddaughter and her very appearance gives me a warm experience of joy. Her smile, her eyes, her calling me "Gandy" all trigger a series of events in my brain that creates a "pleasure construct." We have countless pleasure constructs, triggered by stimuli such as food, sex, a football team, a favorite perfume, and even spiritual experiences. The perception of these pleasures all begin with a gentle, normal release of dopamine in the midbrain.
Drugs initiate a massive release of dopamine, more than the brain was ever intended to handle.
The midbrain tells the rest of the brain that this drug is irrationally good, and floods the pleasure circuit with dopamine. Then something very critical occurs in about 10% of the population, sometimes quickly and sometimes over longer periods of time: addiction.
In addiction, the midbrain hyper-prioritizes the drug. In its primal way, with its massive releases of dopamine, the midbrain says to the rest of the brain that the drug is more important than anything else; food, sex, health, family, God, etc. The drug becomes the primary focus of the brain.
The two brains, the prefrontal cortex and midbrain, should work in concert, with the prefrontal cortex in charge. In addiction, however, the midbrain takes over and overrides the critical thinking areas of the prefrontal cortex. This disconnect explains the destructive, tragic and often incomprehensible behavior of the alcoholic.