Some people get addicted and dependent because they have learned to escape their general life situation by consuming drugs which help them cope with negative emotions. You will find a lot of these people in lower and middle class jobs.
Some people get addicted because they have too much free time, too much cash at hand and are generally a person who constantly requires a lot of stimulation and activity. You will find a lot of these people in executive positions.
MANY people get dependent because they were in an accident, got prescribed oxycodone or hydrocodone, religiously took their medicine according to the prescribed schedule of two pills every four hours for two weeks, and were then denied a refill and possibly labeled a drug-seeker, leaving them very vulnerable to drug pushers. You will find a lot of these people dead on the street.