Alfie Kohn said:
intrinsic motivation – has a huge empirical base of support in workplaces, schools, and elsewhere. We’ve long known that the pleasure one takes from an activity is a powerful predictor of success. For example, one group of researchers tried to sort out the factors that helped third and fourth graders remember what they had been reading. They found that how interested the students were in the passage was thirty times more important than how ‘readable’ the passage was.