Most important, in flow, the relationship between what a person had to do and what he could do was perfect. The challenge wasn’t too easy. Nor was it too difficult. It was a notch or two beyond his current abilities, which stretched the body and mind in a way that made the effort itself the most delicious reward. That balance produced a degree of focus and satisfaction that easily surpassed other, more quotidian, experiences. In flow, people lived so deeply in the moment, and felt so utterly in control, that their sense of time, place, and even self melted away. They were autonomous, of course. But more than that, they were engaged.
― Daniel Pink, Drive: The Surprising Truth About What Motivates Us