Not everyone who uses drugs becomes addicted. Each person's body and brain are different. So people react to drugs differently. Your relationships, surroundings, and stress can also make you more or less likely to become addicted.
But how does taking drugs become an addiction?
Our brains want us to repeat things that we need or enjoy—like eating a good meal. That's why you want to eat more dessert than you know you should. That's why a little child often shouts "again!" when you do something to make her laugh.
All drugs of abuse excite the parts of the brain that make you feel good. But, after you take a drug for a while, the feel-good parts of your brain get used to it. Then you need to take more of the drug to get the same good feeling. Soon, your brain and body must have the drug to just feel normal. You feel sick and awful without the drug. You no longer have the good feelings that you had when you first used the drug.