2 people take the same dose of heroin, they repeat the experience 5 times each on the same time line. Lets say they both has the same surgery. One person stops easily, experiencing mild withdrawal that feels like a flu and goes on with their life without ever thinking about it again. The other feels a powerful compulsion to take more, they maintain their usage say initially through extending a medical script and later the black market.
What was different between the two? Maybe you think person 2 had terrible moral character but if they had never been given heroin this would never have manifested. We call that pathological difference a disease and try and treat it. What would you call it?