He says students are not used to failing. In high school teachers focus on their successes amd they're given lots of encouragement. But when you're doing something you've never done before, you're going to fail and fail until you do it right, and then you'll move on. If you're doing something no one has done before, you'll fail even more before you succeed. In the end, you'll have lots more failures under your belt than successes. In my husband's opinion, if his students are not failing a lot, they're not trying hard enough.
One day I commented to my piano teacher that I was finally starting to enjoy the sound of my own playing. "Don't get too used to that!" he said sardonically. His explanation described essentially the same idea: if we're working to achieve mastery, we're doing something that most people can't do. And we're going to fail a LOT. When we do succeed, we'll just move on to the next thing, and fail at that until we do it right. Then we'll move on again.
If you're not failing a lot, you're not trying hard enough.
Somehow that makes me feel better about my failures.