That cycle is what psychologists dub “the overjustification effect”. It is the name for our tendency to become less intrinsically motivated to do something that we used to enjoy because of external incentives. Basically, the more we do things for reasons outside of ourselves, the less we actually want to do the task.

If we relate that to exercise, it means the more we focus on exercising to look a certain way, to justify food choices or lifestyle habits or to impress other people, the less likely we are to find joy in the activity (and therefore less likely to stick at it).

The phrase comes from a study from the 1970s, in which participants were asked to complete a puzzle task. When they were offered a financial reward for the number of puzzles solved, they were more motivated. But the effect was temporary and as soon as the external reward was removed again, the motivation reversed and their effort levels dropped drastically. Participants actually ended up doing less than before the reward had been introduced.

Workout motivation: the overjustification effect might explain why you don’t want to exercise