Why is it we are motivated to act in ways that are harmful to ourselves? Is it because we derive some satisfaction from behaving badly? What if the consequences of doing so were harmful to your health? Some researchers say that if people are educated about the consequences of their behavior, they will change their behavior. The answer to behavior change in my opinion is more complicated and driven by unconscious motivations.
In the following post on KevinMD, Dr. Laffel explores the question of engagement and behavior change using data from a behavioral intervention program. http://www.kevinmd.com/blog/2011/09/explaining-human-motivation-unhealthy-behavior.html