I think Joker put it well when it said: "The thing about mental illness is that people expect you to act like you do not have it." I have my own idiom: "If these people could be normal, they would chose to be normal."
#1 thing: It feels amazing. Your brain LOVES these drugs. It is like an orgasm mixed with a super Mario star.
There are a few routes: The invincibility of youth colliding with brain chemistry of intense drug addiction. Nobody thinks THEY are the one who is going to have a drug problem. They can quit whenever they want to. Until they stop.
Self loathing is another path. You hate yourself. You don't care about anything. You actively want to destroy yourself, but the drugs make you feel good for a little while. So you keep doing them.
There are obviously more paths than these two.
Either way, you end up addicted. Your brain is screaming for more. You are in physical pain. Every bout of depression and hardship, your brain wills you towards your favorite fix. And in a moment of self-loathing or just plain exhaustion, you cave.
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I am thankful I read the works of William S. Burroughs and Philip K. Dick in my formative years, so I had a stark understanding of which drugs were which, and what happens when you lose control.
I have had the same experience with vyvanse. I didn't find it fun, it would just kill my libido and hunger, to the point where it would be 9pm an I would be a grumpy mess from not eating all day.
I've always loved this passage from "A Scanner Darkly." It is autobiographical:
"Or a horrible positive reason: to have watched a human being you loved deeply, that you had gotten real close to, held and slept with and kissed and worried about and befriended and most of all admired—to see that warm living person burn out from the inside, burn from the heart outward. Until it clicked and clacked like an insect, repeating one sentence again and again. A recording. A closed loop of tape."