It's anecdotal of course, and correlation doesn't equal causation and all that, but the people I'm close to who seem to really have addiction issues seem to be people who had addiction issues a priori.

e.g. I can't even conceive of a scenario where I could find myself thinking about willingly even trying heroin or cocaine. But I have a few family members and (former) friends with addiction problems so severe that pounding a case of beer before snorting lines is what they call a normal Thursday afternoon and heroin is considered a weekend recreational sport. Something predisposed them to the notion that this was a valid idea whereas I can't even conceive of it.