People believe that motivation is the key to behavior change, but motivation is fickle and unreliable. Motivation tricks people into setting overly ambitious goals, then lets them down: this is called the Motivation Monkey. It sets many traps that can frustrate good intentions.

Motivation is complex. It contains three parts: what a person already wants to do, what a person receives in the way of rewards and punishments, and what the context—social, media, and other inputs—does to push or pull a person’s desires. Sometimes motives get into conflict: Someone wants to eliminate sugar but loves cupcakes, for example.

Surges of motivation, called motivation waves, can show up during emergencies or when we’re highly inspired, so that we can make a big effort once. On a daily basis, such motivation will soon fade away, but the initial thrill of excitement or fear causes us to overestimate our ability to commit. A person’s desire to accomplish things changes from hour to hour and day to day: People tire as the day progresses, and things that seem vital on Monday may look unimportant on Friday. Diets famously collapse during the holidays.

Some aspirations are fairly constant: Be healthy, be less stressed, do what we should do.