I have learned to keep students’ repertoire to a level that will challenge them somewhat,
but not overwhelm them. Once in a while, I have miscalculated, and have had to suggest
putting the difficult piece “on hold” until they have had some time to build better
technique. If it is a piece that the student has chosen, and really wants to play, this
approach guarantees that they will still get their wish. You are absolutely right that it is
better to use pieces that the student loves to play. If, however, you are teaching a very
serious student who is planning a career, then they have to learn every kind of repertoire.
whether they like it or not. To play a piece well, when you have no emotional connection to
it, this is like great acting, and it takes practice.