Naruto first realized the kind of ninja that Sasuke was by his kisses. They were quick and fleeting and perfectly placed and felt somewhere between dangerous and addictive. Sasuke's kisses were like Sasuke himself. He was passionate beneath it all -- ruling his emotions s he wouldn't be ruled by them.
Naruto, in turn, was a bit clumsy. He had power, but lacked control, and Sasuke seemed to love it despite his protests. Otherwise, why would he keep up with the kissing?
They never discussed it. It had started back before the chuunin exam with a case of Sakura misplaced. From there, Naruto's clumsiness had a big part. During chakra training away from Kakashi's view, they had raced up the same tree. Naruto's foot had slipped and Sasuke was immediately there, his lips somehow realigning Naruto's chakra for him and keeping him stable even as his mind raced while trying to come up with something to say. By the time he decided to stick to the usual insults, Sasuke had gone. The next day, Naruto fell out of a tree six seperate times, five of them while trying to keep an eye on Sasuke. Only once was for Sakura.
When Naruto sliced his finger with a kunai, Sasuke cleaned up the wound and kissed Naruto's lips and neck to the point of utter distraction while he bandaged it. This kept Naruto from noticing the word "idiot" which appeared on the bandage once Sasuke was through with it; he didn't notice for three days.
Sasuke grabbed the end of one of Naruto's ramen noodles at lunch one day and slurped it so his lips just touched Naruto's before he bit at the noodle, lightly scraping Naruto's bottom lip with his teeth in the process. He acted as though nothing had happened, and Naruto came to follow suit. Kisses became commonplace, but still retained that danger and passion they'd had from the start.
Sakura always seemed to feel they didn't get along, but to them the antagonism they shared began to give them reasons to make the kisses deeper. They made each other mad just to drive themselves further into the spiral of lust they'd developed as a whole new kind of training.
Naruto learned to pool his chakra in his lips to make the kisses even better. When Sasuke left, Naruto started to keep up the practice so that, once in a while, he could feel the tingle because while it wasn't anywhere near the feeling of Sasuke's lips, it eased his mind as though he was ready for when Sasuke would come back and kiss him once more, someday.