Zuko noticed that in all their family portraits, they never smiled.
His father's stern look pierced the very paper and ink it was born from, shadowing the real-life version that inspired it. His face was marble; cold and harsh. His sister's cruel, calculating gaze showed no signs of happiness, no hint of joy. Her lips stayed thin and focused, her face devoid of any childlike wonder. Zuko sat beside her, his own face marred by the solemn faces of his family. And his mother hovered above her children, her warm eyes dulled and her bright, soothing countenance spoiled by the oppressive atmosphere that their family somehow always created. Zuko remembered the bitter, sour air that filled the room whenever they all sat together, as if something terrible always loomed on the horizon. But what bothered him most of all was that they never smiled. Though the happy moments were few and far between, there was no record of them, and no proof they ever even happened.
That's why, when Zuko sat with his wife and the court artist, he turned to her, brushed a strand of ink-black hair out of her eye, and told her to smile.
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Just a quick idea that came to me while watching The Beach. Written in 5 seconds, so I apologize for the quality.