This jives with my experience using pretty much everything, including heroin. A high-level unscientific explanation I've had is basically folks prone to addiction feel like heroin or whatever replaces the feelings of success and companionship etc. I made it most of the way through college being an occasional user and passed up opportunity's to get shit because I was focused on a project or had hope of something happening that was exciting in the short term. This is probably a-typical. A lot of guys I knew are dead or fell in without much of an interest in other things. Like they had poor prospects in life.