Emotions are not super well understood, and I think they are at the root of addiction. If you are chronically unhappy, drugs let you feel good. You start feeling better because of the drugs, and lose the ability to respond to negative emotion in a healthy way. Healthy emotional balance lets people make good choices and fix bad problems. Being chronically unhappy - perhaps as a result of unresolved mental injury - will hamper a persons ability to make good choices, just like being keep in darkness for a log time will make your eyes way too sensitive to handle normal light.
Show me someone who is addicted to drugs, and I'll show you someone who is unhappy about something other than the drugs. The addiction just makes the underlying problem worse.
If you can learn to fix the underlying problem, then fixing the addiction itself is much easier.
Once you are knee deep in drugs, your life becomes awful which gives you more reason to keep using the drugs. It's like a local minimum, or an AI that learns to manipulate it's model of the world, because that's easier than manipulating the world itself.
I suspect that actually curing addiction, or many other mental diseases, will require not just changes within, but also changes externally.
The sooner society can rid it's self of the blame culture, and adopt the more pragmatic 'solve the problems' culture, the better.
One could argue, all that means is that "it" is addiction that one can't manage on one's own.
However I've started to come to think of addiction notice a disease onto itself, but as a symptom of some other underlying "dis-ease", clinical or otherwise.
If anything, the debate over whether addiction is a disease underscores the fact that the people having the debate have a very narrow definition of "disease". If addiction isn't disease, then what is it?