What Causes Teen Substance Abuse?

In Los Angeles and across the country, it seems that far too many teens are admitted to drug rehab centers each and every year. But what are the factors that cause this to happen? After all, there are many anti-substance abuse campaigns directed at teens, and most at least somewhat understand the dangers of drug and alcohol abuse. However, there are also several factors that can make it easy for teens to give in to the temptation to use drugs or alcohol.