Basically people use substances such as alcohol and drugs because they like the way the substances make them feel.
Pleasure is a powerful force.
If you do something that gives you pleasure, the brain is wired in such a way that you will probably want to do it again. All drugs that are addicting can activate and affect the brain's pleasure circuit. Addiction is a disease that affects your brain and behavior. When you become addicted to alcohol or drugs, your brain actually changes in certain ways. Someone who is addicted uses drugs without regard to the consequences, such as problems with health, money, relationships and performance at work or at school.