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1. Their first kiss is years before their second. Just Naruto tripping over his shoelaces and falling into Sasuke, and startled wide blue eyes before the briefest contact of lips. Sasuke doesn't remember it well and spends far too much time trying to recreate it in his head, which is his first clue.
2. The first compliment Sasuke ever gives to Naruto is also an insult to all of his friends. "Why are you defending weak people?" Sasuke says, scathingly. "You're not one of them." It takes Naruto a week to realize what it is, and by then he has already gone and punched him and gotten in detention.
3. Their first date is a fight. Sasuke calls him a coward, and Naruto holds onto his temper — see, Iruka? — long enough to challenge him to a fight after school. Sasuke smirks and mutters, low and smug: "It's a date, Uzumaki." Naruto stares and sputters and feels like a fire hydrant.
Later, after the fight, when they're both bruised and panting, too tired to move, almost touching in the grass, Naruto starts talking. First about what an asshole Sasuke is, and then about his dad and his friends and the color orange. Sasuke doesn't say much, just interjects an insult to Naruto's intelligence, personality, or tastes every so often, but by now Naruto realizes that the insults are like punctuation; a necessary component of speech for Sasuke.
When Sasuke finally stands and dusts himself off, Naruto yells, "Rematch tomorrow, bastard?" before he quite thinks about it. "If you want to get your ass kicked so badly," is Sasuke's parting shot, but there's no sting to the words.
They meet up every day for the next month.
4. Naruto's first apology is for skipping their fight to play laser tag with his friends.
Naruto read somewhere once that exercise is supposed to release endorphins. This must be the reason why he's so pumped up and re-energized after their fights. But after laser tag, he's only feeling vaguely guilty and sorry. He tries not to imagine Sasuke alone on their hill, tries not to wonder how long he waited before he left.
Sasuke doesn't show up after school for a week. It's the worst week Naruto has had for a long time. He can't sleep properly and he's full of too much energy, something pent-up and bubbling inside of him. It explodes over in snappish remarks and general irritation with the world. When he blows up at his dad over dinner one night, he decides that enough is enough. He's not sure when collecting aching bruises and stinging cuts became the highlight of his day, but he knows that he doesn't want to lose this, whatever it is between them.
The next time he sees Sasuke in the halls he punches him, not hard, just to get his attention, and mutters, "Sorry I'm late, bastard." Sasuke punches him back, harder than necessary, and Kakashi gives them detention for a month. It's worth it.
Things don't go back to the way they were before. They move forward.
5. Their second kiss is deliberate. It is also right before the first time Itachi beats Naruto up. (Itachi claims that he was merely carrying out his duty as older brother and defending Sasuke's maidenly virtue, according to ancient Uchiha courting rituals.) Sasuke pulls Itachi off Naruto and yells at him, but secretly he's pleased. This is the most brotherly bonding they've done in years.
6. Sex is easy, compared to talking. It's almost like fighting, and Sasuke understands fighting. Words are harder. Naruto is full of them, so he just tosses them out and waits for the right ones to stick. Sasuke struggles. In the meantime, he touches Naruto and smiles and hopes that his thoughts will diffuse through skin to skin contact.
7. The first time Sasuke says "I love you" is not during sex, though his heart is pounding in his ears anyway. It's standing outside Naruto's apartment, three in the morning, right before their goodnight kiss (which turned into Sasuke being dragged inside for goodnight sex).
He manages not to stutter only because he's been practicing this to his mirror for weeks.
(Itachi caught him once and made dry jokes about narcissism until Sasuke explained, when he got very, very quiet instead. Itachi's a lot nicer to Naruto now, which means that when Itachi catches Naruto touching him he only glares and mutters silky death threats instead of punching him. This is progress.)
8. Naruto asks Sasuke to marry him years before it's legal. Naruto is convinced that it's only a matter of time before they can, and it doesn't even occur to him to wonder if they'll still be together then. He knows it, like he knows the sun will rise in the morning.
9. He's right.