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Tear Jerker: Avatar: The Last Airbender - The Promise
Aang and Zuko's talk during the fireworks, while everyone else is having a good time Zuko goes and puts poor Aang on the spot with his vow to kill the prince should he ever get out of hand.
Aang severing his link with Roku and crying a Single Tear, signifying that he knows this is what must be done, but still feels conflicted about it. While Roku urges Aang to make harsh decisions, even with regards to his own great-grandson, he's essentially trying to help Aang avoid his mistakes, and it's sad seeing him call out to Aang one last time before he's cut off.