The production and utilization of the neurotransmitter dopamine causes us to feel good, and dopamine is one of the keys to addiction. Because of a small difference in the 11th chromosome between an addict and a non-addict, dopamine production and utilization is lacking in the addicted brain. To put it simply, addicts are born without the ability to feel as good about themselves as non-addicts.
In the normal brain, a certain cascade of chemicals occurs in the limbic system, ultimately resulting in enough dopamine and enough dopamine receptors available to use that dopamine, which gives a feeling of well-being and reward.
In the addicted brain, the cascade does not proceed properly, and either there isn't enough dopamine available or it cannot be used, leaving the person deficient. This is called Reward Deficiency Syndrome (RDS). The RDS person learns over time that other chemicals or activities (alcohol, cigarettes, drugs, sex, food, etc.) will give the feeling of reward that (s)he is lacking
Isn't it funny that these substitutes are often called "dope"? I'm such a weirdo for even considering that, aren't I. I wonder if God giggles at stuff like this.