It's basically the tried and true idea that progress and growth comes from difficult exertion of one form or another. If the muscle isn't ever tired, it probabl

It's basically the tried and true idea that progress and growth comes from difficult exertion of one form or another. If the muscle isn't ever tired, it probably isn't getting stronger. If there is no uncertainty in how to solve any parts of the problem, one won't learn as much. Fortunately, difficulty is often correlated with interestingness (not always though).