For myself i never thought i could be addicted to anything. The only reason why I even tried drugs was because someone i thought i had feelings for was into to drugs. Our discussion that night wasn't just about what makes people start taking drugs, it was also about what makes them get addicted to them. You might try the addictive substance because you hang out with people who do it, but to become an addict you most likely have deeper personal issues. Most probably psychological problems caused by parental issues or other problems in your childhood or adolescence.
Drug addiction is a form of escape or even self abuse.