Habitual Thinking

Where do peoples' minds take those tangents that people take from time to time, the ones like trying to quit smoking, or trying to get six servings of vegetables a day, or other healthy and productive habits, that lead one down the path of temporary enlightenment? Why is it that it is so hard to follow through? People set goals, do those worthwhile endeavors for a little while, and then stop. What I'm wondering is, what is going on in the brain to make this happen? And to make it stop? Is it a brain problem? Too hard to make the mind bend to new proportions? And why is it so much easier to fall into bad habits than into good habits? Is it because the pleasure part of the brain is more developed?