Why do people get addicted to drugs or alcohol?

People become addicted because drugs and alcohol (a drug) cause changes in brain chemistry. To begin with, that just makes us feel good. Eventually, the drug no longer works as well (tolerance), and we need more to feel good. We feel bad if we don't have them. Eventually they become necessary for anything like "normal" functioning.

When we need drugs just to feel normal, we are addicted, and withdrawal usually makes us feel so bad that we convince ourselves that we'll quit later -- or that we don't have a problem. Things go downhill from there.