For one of my students:
Mason had a short temper all through school, and got in trouble a lot for reacting without thinking. When he was at his middle school one day in 7th grade, another student tried to pick a fight with him - called him names, called his brother and then his mother names, and finally took a swing at him. Mason stood there and blocked for nearly 5 minutes until a teacher showed up. Not once did he try to hit, kick, or throw the other student, and not once did the other student hit him, other than the blocks. This was a turning point for Mason, and changed his whole view of himself as incapable of keeping his temper - he never got in trouble for that again. When asked how he did it, he said "I just did what you and Mr. Miller (the other instructor) told me to do"
For another:
Josh has diabetes. His doctor wouldn't clear him for his learner's permit when he was 15 because the doctor felt he didn't have sufficient control over his blood sugar. After about 6 months in my class, he told me that his doctor had finally cleared him. He went on to say that the discipline he learned in class was what he needed to learn to monitor and react appropriately to his blood sugar, instead of relying on his parents (he was 16 by this point).
I could go on... things like this are why I teach.
For myself, it had to be the look on my instructor's face when I was testing for IV Dan, and, when asked what my technique break was, said high twisting kick (head high, ball of foot, basically kicking back over your own shoulder). It took me 6 months to work up to that level of flexibility and strength at that height - but it was worth it for that one expression.