As someone who has struggled with being overweight/over-eating I understand that addiction is tough. It's the only "disease" that can be fought and won by making better choices. My wife's cancer couldn't be willed away by avoiding her cancer dealer. My son's seizures aren't a product of overindulgence. I understand the "condition" but calling it a disease drives me up a fucking wall. Then again, I'm just a lowly street medic (mediocre at that) and not a psychiatrist or behavior specialist.
Look into the neurobiology of drug addiction, read it, study it and then you'll understand. Also, look up the actual definition for disease. We really need to be careful to separate the initial decision and the disease. They are two very different events.