I think you're oversimplifying things a little bit. Some people will try e.g. heroin and get hooked due to curiosity. Some people do derail otherwise promising lives with drug and alcohol use.
Edit: And of course it doesn't require that they have obviously impoverished hopeless lives. Part of the illness of our society is the huge numbers of depressed/lonely/etc middle class people who otherwise seem to have a life "on track"