Maturing
Teenagers are a strange breed of animals. Privately, Kisame is endlessly amused, and not particularly annoyed. He still remembers being 14, running around doing such incredibly stupid things without thought for the consequences or even his own mortality, joining up with the most ridiculous bands of outlaws with that childish conviction that whatever he was doing was undeniably noble and right. The childish notion that he was immortal, the smartest, strongest, most righteous, no matter what anyone else said or how much he was beaten.
This is what he expects when he looks at children. Genin are so easy to predict nowadays, when he's old enough to look back and laugh at them. Kisame doesn't particularly mind the younger members of Akatsuki. Unlike Sasori, who is always endlessly impatient with his own younger partner, grouching on about how the boy lags too much, is always running around showing up late and then taking off into the sky in a heartbeat without any sort of thought for the consequences and all those other things that to a 17 year old fresh out of his village must seem so endlessly boring.
Kisame puts up with it all remarkably well, enough that Sasori has occasionally mentioned that he should just take Deidara with him and leave Sasori with Itachi, who to Sasori (although annoying in his own way, but then with Sasori everyone is) is the epitome of maturity 99 of the time (privately, Kisame wonders if this is partially because Itachi has spoken all of ten words to Sasori in all their years of acquaintance).
Kisame takes this as a joke, he doubts even Sasori would actually switch partners so easily, and for all of his annoyance Sasori and Deidara get along better than many of the others. Besides, Kisame wouldn't switch Itachi for Deidara if he were paid to do so, it seems like exchanging gold for a lump of coal.
Itachi is probably the least teenage-like boy Kisame has ever seen. He still remembers Itachi at 14, newly made Akatsuki member and assigned to Kisame (sometimes Kisame wonders if he weren't assigned to Itachi precisely because he puts up with children well). That serious little boy was hardly what he'd expected, the description had been "14, short with black hair, an expert in genjutsu and some sort of special Sharingan". No one had said how quiet Itachi would be, and initially Kisame had taken it for shyness and made himself look like a fool trying to get the kid comfortable. He still remembers practically tripping over himself trying to reassure Itachi that the Akatsuki didn't expect him to rush into S class missions alone all of a sudden, and that he would be around all the time just in case anything went wrong.
After seeing Itachi in action, he had spent a long, long time wondering exactly what the boy was. Not a teenager, certainly not 14 years old. That permanent Sharingan that he'd taken to be an overly enthusiastic precaution started seeming more like a negligible habit, the silence became less shyness and more a disconcerting amount of that quiet self-assurance which required no encouragement. At first, Kisame had began to wonder if perhaps children weren't supposed to act otherwise, and the others simply lacked training. But after a few weeks of Itachi's company on the road, watching the boy go to sleep armed to the teeth and still in his dusty traveling cloths, ANBU-like in his caution, Kisame had realized that Itachi was the deviant, not the other way around.
After all, no fourteen year old should discuss anything and everything in such a bored monotone, as if he'd done it all before. Teenagers weren't supposed to talk about sex in such a straightforward manner. It was and probably always will be an unnerving experience to look at Itachi's too-young face and then listen to his voice, like an incongruence that was (albeit very slowly) going away now that Itachi couldn't be mistaken for a child anymore.
Kisame feels guilty about thinking it, more because of team loyalty than any particular regard for Itachi himself, but sometimes he wonders why he doesn't just take Sasori up on his half-serious offer. Deidara, he thinks, would be the only one to really protest. That boy is ridiculously attached to his partner. Kisame thinks that Sasori's gruffness only eggs the boy on to try harder to get closer to him, as if it is a challenge. He wonders sometimes what it would be like to work with Deidara. Certainly annoying at times, but Kisame can take it all in good humor. Deidara is easily excited, loves making a scene, blowing stuff up, often without particular discrimination. He thinks he can even find the sculptures and all that art-talk stuff interesting, if he tries hard enough. Certainly it can't be any harder than putting up with Itachi's strangely silent mood swings.
Most of all, though, is the idea that Kisame is comfortable with Deidara. The boy is weird, sure, and bordering on megalomania at times, and occasionally Kisame has heard of him doing some spectacularly stupid things on a mission, but all of that is expected and even welcome in a way, and Kisame (unlike Sasori, who is convinced that a moron will always be a moron) knows Deidara is someday going to grow up, stop with the frivolous explosions, maybe even become a grouch like Sasori.
Kisame isn't so sure about his own partner. Itachi to him is as much a stranger now as he was when they met. Oh, Kisame knows his moods better, and he can tell that some things are going to annoy Itachi and some things please him, but that doesn't make the boy any more understandable. His motives, even his reasoning pattern, as just as foreign as ever.
Sometimes Kisame wonders if Itachi is going to grow up someday, what he might become.
All in all, it is not a pleasant thought.