I'll chime in a bit late. I tried a number of substances in my youth, looking for whatever it was that I thought would fill what I perceived to be a hole in my personality/life. Those things varied from reasonably harmless, socially acceptable substances to things that could have been very addictive and quite dangerous. If my psychology were wired for addiction it certainly could have happened, but I'm grateful that the only dependency I've ever developed has been caffeine (and that, my friends, is still one mean monkey on the back).

Long story short, I think a lot of people who try heroin, crack, meth, or whatever are trying to fill a hole. They're just searching and for whatever reason would rather put something in their bloodstream than try to knit a sweater or work in an animal shelter. I am very far from an expert, but the psychology of dependency, abuse, and addiction is complicated and what causes a problem for one person will be just a passing thing for another.