It's not like cancer. Cancer isn't a disease where you can really participate in it's life-long treatment.
Alcoholism is like diabetes. You need to treat it daily and it's tempting to consume stuff that is going to really hurt you and ultimately COULD cause death. (I can actually see a person offering a diabetic a bite of cake and asking what harm a bite would do. Seems very similar to alcoholism. In my opinion anyway.)
There ARE people out there with type 2 diabetes that continue to eat themselves closer and closer to death. I imagine that their loved ones have felt a pain similar to the that of the people who love alcoholics. It's hard to watch someone make poor decisions for their health over and over again.
ETA - And DD, good for you for having some really solid boundaries! YOU are making good decisions for yourself and that's the absolute best thing that YOU can do. I think active alcoholics confuse compassion with sympathy. I don't feel sorry for my AH, I just know what it feels like to feel kind of helpless. I'm sorry that he's hurting, I've been there and I know that there are many healthy people who care about him that would love to help him and see him be healthy. We support sick people by not losing our minds because they've lost theirs.