Warmth
Disclaimer: I own as much as Jon Snow knows
"Zuko, you're really warm."
"Really? As a firebender, I hadn't noticed."
"Shut up Sparky," Katara poked him, ignoring his yelp of protest. "What I'm saying is you're really warm, borderline hot–"
"You've never brought this up before."
She raised her eyebrow. "I'm pretty sure I have."
"Alright, let me rephrase that; you've never complained about this before. When I've stayed in your room in the Southern Water Tribe, and also when we've been camping in the Earth Kingdom, you've always insisted on cuddling next to me in bed, among other things, saying on many an occasion 'oh Zuko, I love how much warmth you generate, ooh, I'm going to be so nice and toasty tonight'."
"Well for one, I don't talk like that, but that's a legitimate reason! The South Pole is freezing, and parts of the Earth Kingdom can get cold at night too, therefore I need your warmth. However, the opposite is true in the Fire Nation, and while it's mostly tolerable, there are some nights, like this where it's just too much! I'm pretty sure most of the warmth is being produced by you, though."
"Yes, as a firebender, I'm naturally warm. I don't know if this is a waterbender thing, or just you, but you're naturally cool. I'm going to sleep now."
Katara pouted, before an evil idea popped into her mind.
"Katara, what are you – sweet Agni Katara, your feet are freezing! Get them off me! Get them off – argh!"
"Oh dear Zuko, you appear to have fallen off the bed. Must be a firebender thing."