A speaker at my treatment yesterday put in a simple way that I identified with:
Alcoholism is an obsession of the mind and an allergy of the body. When I'm not drinking I'm constantly thinking about it. The allergy is that once I have one drink it's nearly impossible to stop.
And that was totally me, and I think it applies to anyone who is an addict. To treat this takes much more than simply ceasing to use the substance, but to treat the underlying obsession.