... a clinical psychologist ... dealing with individuals who for other reasons had a predisposition (for psychological disorders) and/or subsequently self-medicate and/or were led into living in a sub-culture where drug abuse is rampant, as they didn't "fit" and were ostracised (even if only by their own perception) by "normal" society.
I believe that there's also an element of protest: "I'll do it *because* it's illegal, as way of showing the Man that I'm not totally in his power". The same goes for lots of other anti-social behaviour.
I'm sure you're right. I was an appalling teenager - a parent's nightmare of a rebellious rude uncooperative and anti-establishment (often "automatic" rather than a reasoned reaction) young person. I got through it relatively unscathed due to a lot of luck.