The answer to this question is different for each person. However, there are some pretty common scenarios that can occur.
Sometimes, an individual witnesses drug and alcohol abuse from adults around them while they are growing up. In this type of situation, a person usually feels as if they never had a chance in the first place. This is often true for children and young teens who grow up in a drug and alcohol filled environment.
For others, substance use starts as a result of curiosity and experimentation. A teenager, for example, may start drinking with their friends at parties. As time goes on, that teenager and their friends will start to hear rumors about other drugs. The teenagers may begin seeking something that will make them feel better than alcohol. If the opportunity arises, they are likely to begin experimenting with drugs. By the time they realize what has happened, they are addicted and feel there is no turning back.