To me the hard part of teaching is that more effort, in terms of getting students to learn, isn't always appreciated by the students. A teacher who's easy and funny, but doesn't actually get them to learn much, is usually seen as a better teacher by the students even if they're not actually a better teacher. Thus, working hard can sometimes feel unrewarded. Teachers who have a high level of intrinsic motivation thus get burnt out over time.
To me the hard part of teaching is that more effort, in terms of getting students to learn, isn't always appreciated by the students. A teacher who's easy and funny, but doesn't actually get them to learn much, is usually seen as a better teacher by the students even if they're not actually a better teacher. Thus, working hard can sometimes feel unrewarded. Teachers who have a high level of intrinsic motivation thus get burnt out over time.