Only certain people should drive Jeeps or indulge in mind-altering vices. If you're so ugly that daylight refuses to come in when you open the curtains in the morning, you shouldn't get behind the wheel of an open vehicle. Likewise if you have an addictive personality, you shouldn't partake too heavily in drugs and alcohol. People like this seem to do things in excess without knowing their limitations. Call them extremists.
Emotional state is another factor that can affect drug use.
If someone is depressed or angry they may turn to drugs to get through the rough times and then have trouble living life without them. Even if you don't get physically addicted, you can still get mentally hooked. Abuse over time leads to tolerance, which brings on dependence. Physical dependence is bad. That's where you have to use the drug just to feel normal for a few minutes.