This is true on its face for anyone who's spent hours and hours trying to beat a tough level of a video game -- we even call it "grinding" (at least in some contexts, namely RPGs), and to some extent it's not really "fun," but it also has a satisfying and compulsive aspect that drives you onward despite the challenge. I can't recall public education ever stirring the same sort of feeling in me (and I did quite well in school).