"Why doesn't s/he just stop when s/he knows how much it's hurting me?" This is one of the most common questions asked by those who love someone addicted to substances and/or harmful behaviors.
It might be helpful for those loved ones to know that stopping isn't simply a matter of wanting to, or simply understanding that you may be causing someone else's pain. Research indicates that to "just stop" is not so easy.
One of the main reasons is called dopamine. Research on the brain indicates that addiction is about powerful memories, and recovery is a slow process in which the influence of those memories is diminished. Both addictive drugs and highly pleasurable or intense experiences (such as a life or death thrill, a crime, or an orgasm) trigger the release of the brain chemical dopamine, which in turn creates a reward circuit in the brain. This circuit registers that intense experience as "important" and creates lasting memories of it as a pleasurable experience. Dopamine changes the brain on a cellular level, commanding the brain to "do it again," which heightens the possibility of relapse even long after the behavior (or drug) has stopped. Dopamine also helps to explain why intense experiences can be just as addictive as drugs.
Additional research on addiction indicates that dopamine is not just a messenger that dictates what feels good; it also tells the brain what is important and what to pay attention to in order to survive. And the more powerful the experience is, the stronger the message is to the brain to repeat the activity for survival. Additionally, those who have fewer salient things in their lives that capture their interest and attention are more vulnerable to those things that may give them a rush and alert the brain in a powerful way.
This research on dopamine goes a long way in explaining how someone can become addicted to something that can become so destructive and detrimental in their lives and the lives of those they love. It also helps to explain why meditation, yoga, exercise, and acupuncture can be helpful tools in the fight against addiction, as they address the physiology and biochemistry of the individual.
Battling addiction is not simply a matter of willpower, but also is about transforming an individual's body, mind, and life and creating a new set of experiences for the brain to register as important and pleasurable. It is also about patience, healing, not taking relapse personally, and the passage of time to allow the memories to fade.