If you're a game designer for long enough, eventually you'll work on a game that ends up just not being all that fun. (No matter how great you are, everyone has their mediocre projects.)
As a teacher, there are analogous cases. Eventually you'll meet a student who you know is capable of grasping the course material, and they genuinely try hard, but for whatever reason they just don't seem to get it. Or, eventually you'll create an exam or assign a homework and half the class completely bombs it.
In either case, there are two types of people. There are the ones that look at the tiny amount of positive feedback (the one glowing game review, the one student who gets everything perfect) and decides that if this tiny minority understands what they were trying to do, then everyone could, and it's just that the others aren't trying hard enough. It's certainly not their fault if these people just don't put in the effort to see their obvious brilliance. They feel better, but they don't actually get any better.
Then there are the ones who learn humility. Yes, others may have made mistakes along the way, but something in your own work went wrong as well -- enough that it was unable to save everything else. By figuring out what you did wrong, you become stronger.
In games, this is actually pretty easy. Reviewers will quite happily lay your game on the table and dissect each of its flaws, for all to see. Post-mortems, likewise, show us that most development teams are painfully aware of their issues; if you as a designer have made mistakes, even if you don't know what they are, someone else on your project team certainly does.
In teaching, it's much harder. There is no "project team" -- in fact, it's rare to have even a single other education professional observe your work and offer any constructive feedback at all. Students can help you identify weak points, but they can't tell you what to do about them. Teaching, then, forces one to go beyond humility into the realm of self-reflection in a way that game development does not.