Dependence, whether physical or psychological, is defined as a perceived need for something. Of course, the body does not need alcohol, heroin, meth, or other drugs. In fact, the opposite is true, and this is why it is called a ‘perceived need.’ The perceived need for a substance exists often in direct contradiction to rational thinking.
Drinking and doing drugs not only affects the pleasure center of the brain. The reward system is also associated with something called reward-related learning, via dopamine’s interaction with glutamate. Reward-related learning is a very important function for survival. It associates activities that are necessary for staying alive, eating for example, with pleasure.
When addictive substances are used, this same function is engaged. This results in the association of these terribly dangerous substances with general survival. The perceived need emerges.
Most people understand that a drug like meth is bad for you. It is actually neurotoxic, which means it destroys important tissues in the brain. You could tell someone who is addicted to meth this and, while this truth is terrifying, it would not make the person stop doing meth.
The perceived need of a substance supersedes the logical or rational understanding that drugs are bad.