Mental health issues may have no clear physiological cause. Even if there's a physiological aspect, it's unclear what the causes that condition. Depression, for example, we see low serotonin levels, but we don't know why the levels are low. In the case of addiction, the mechanism is clear. https://science.sciencemag.org/content/278/5335/45

Addiction issues are mental health issues because there is no reward; it is not a rational choice. It follows that people with addiction issues are either not informed enough to make a rational choice, irrational, or for some reason are unable to say no.

If the model were really this simple, relapse would be non-existent. I can't believe any former addicts would make a rational choice to relapse.