Every time she pecked his cheek, each time she caressed his face, and all the times she wrapped her arms around him, Zuko died a little inside. The young fire lord cursed himself for it, berated himself as selfish, scoffed at himself to even dream of such a thought, that he would ever be worthy of any woman's love and devotion, let alone Katara's. Not to mention, Katara was with Aang, Zuko's best friend, the avatar. Even with a shot in the dark, he knew he didn't stand a chance.
But, every time they kissed and he was in the room, watching as they put on an overdisplay of affection, calling each other "sweetie" every other word, he thought his heart would fall apart, and he'd throw it up. He felt like they were doing it to spite him, but none of them could have the slightest inkling that he was madly in love with the waterbender.
Zuko would never confess it to her. He knew she would never reciprocate his feelings, she was conspicuously in love with the avatar. She had only forgiven and accepted him by the skin of his teeth. He had hurt her too many times. He would shatter her happiness, most of all. He would shatter Aang's happiness. They were both blatantly happy together. He would ruin two of the dearest friendships he'd ever had, he would lose people he loved out from his life, not to be there at all, and that would hurt more than this. She was so gorgeous. He was so… so… Zuko. He had nothing to offer her, and she had everything to give him. It wouldn't be fair, not even close. Anything he tried, and would lose in the endeavor, would be futile and in vain anyway. None of it was worth it. So he sat there, faking a smile, clenching his teeth, holding down the lump in his throat heavily, his heart sunken into his stomach, aching—
Forever to keep his undying love—
hidden.