When things are in balance, the status quo is met, and the brain repair system appears content. But when things get out of balance, the repair department comes to life and reacts quickly. Drugs work in the brain by mimicking natural chemicals, called neurotransmitters. By imitating these chemicals the drugs can override the system and keep pleasure centers and other regions active much longer than normal. But the brain will eventually try to correct this error.
Some brains respond to the imbalance quicker than others.
The first line of defense in the brain's repair department is to reduce the amount of chemical that the brain produces on it's own. That reduction leads to the drug user's perception of tolerance. Since the brain has compensated for the increase of artificial chemical by reducing production of natural chemical the drug effect is reduced and the user has to increase the amount used. The second line of defense is for the brain to reduce receptor sites, or locations on nerve cells that chemicals can attach to. If the chemicals can't attach, they can't work.