You need to add "habits" to the equation as well. All of those things exhibit the same properties. We differentiate them by a mix of how many problems it causes for the person, for the person's surroundings, and various moral preconceptions about what one should do. Biochemical changes and responsiveness to raw willpower aren't really useful metrics, because both habits and addictions affect the brain, and the whole point of a good habit is to make something that's resistant to willpower.