To make the comparison to Avatar again (full of spoilers), Zuko lost his ability to firebend when he was so focused on using his anger to do so. I had a lot of criticisms about that storyline, because he had a right to be angry (so did Azula, and her anger was presented as an uncontrollable madness) but it was a kid's show, whatever, nuance. I don't understand why anger is so often portrayed as a negative emotion that drags you down, makes you less effective as a person. There's nothing wrong with being angry when it's appropriate.