While addiction is not cancer, making the "choice" to quit is not nearly as easy as it seems. At the point that addiction has taken hold, forces beyond simple will-power are required to reverse the drastic chemical and biological changes that substance abuse has made in the brain. Once an addict, the drive to use chemicals is as strong in the addicted person as the drive to eat, sleep, and have sex is for non addicts.
This paper by Dr. Garrett is probably the best explanation I have read about why, in the midst of their lives falling apart due to addiction, addicts have such a difficult time giving up the very thing that is causing them such misery.
The Addict's Dilemna
As I reread the paper, I noticed this interesting comparison to cancer treatment:
"Addiction is a process that over time encroaches upon and over time invades the normal, healthy "tissue" of the addict’s personality in a manner strikingly similar to the way a malignant tumor crowds and infiltrates the tissue around it. And just as in many cases the Dilemna for the treatment of a bodily cancer is how to remove or destroy the cancer while simultaneously sparing as much as possible of the nearby non-cancerous and often vital host tissue, so does recovery from advanced addiction require a similar separation of "tissues," with destruction of one and protection of another. The process of recovery from addiction in fact quite often resembles the radiation treatment and chemotherapy of a grave malignancy during which the individual often experiences side effects and feels quite ill from the treatment."