Full disclosure: If you buy the book through the link, it won't cost you any more, but I'll get a few pennies. But my main motivation for doing this is to understand the book properly. What follows is my summary of the chapter.
We like to think that we chose our habits on purpose. Yeah right.
Habits build up slowly, like learning to back a car into a tight spot, and we're not always aware that a habit is forming. We tend to assume that we acquire habits as a means to an end. That includes bad habits, although the goal might not be a good one – like getting drunk to forget that you’re an alcoholic.
But sometimes it's the other way around. If your daily walk in the park goes past the ducks, you probably think you do it because you like the ducks. You'll think that even if you did it at random the first time, then the second time you did it because you did it the first time, and the third time because you did it twice before, and so on. The thing is, you're not lying; you really believe you chose it because of the ducks. It's rather like being talked into buying a more expensive car than you wanted, and then convincing yourself that it was worth the extra money. Or changing your mind on an issue, and then being convinced that you always held the view that you've only held since last Thursday. We all want to be right, especially about ourselves.
Of course in real life our habits are some combination of intentions and past behaviour. If a habit is weak (you don't do it that often) you tend to do what you intended (drink more water, watch less TV, whatever). But if you have a strong habit, like eating fast food for lunch every working day, chances are the habit will win. Bizarrely, people with strong habits tend to be more confident that they'll change even though there's much less chance that they'll actually do so. We feel we have more control exactly when we have least.
The main difference seems to be the frequency. If you perform the habit a few times a year, you can probably change. If you do it weekly or more, it's going to be harder. If you do it every time you're in that situation (sit in car, turn on radio: walk into Starbucks, order a latte) it's going to be much harder.
For years psychologists have successfully changed people's intentions. People happily committed to low fat diets, wearing sunscreen, getting more exercise etc. They just don't follow through.
Actually many of our habits are fine. We formed them (or our parents formed them for us) for good reason. It's a darn good idea to brush your teeth, and to look both ways before crossing the street, however preoccupied you are. But we tend not to notice these habits. Instead, we notice the minority of habits which are creating trouble.
You haven't failed to change those habits because you're weak. It's because you're human.