It was a long time coming, and in the end, he wasn't so surprised. Kakashi had noticed Obito since their Academy days. Obito was the chronically late kid. The kid who never remembered the rules. The kid who never had the answers. He was all noise and bluster, but functionally useless. He was all these distasteful things and more, but Kakashi couldn't just leave him to flounder. He had to rub it in.
He felt some guilt at that, really-it wasn't in his nature to be abrasive, or so vocally dismissive. Normally he'd have ignored a kid like Obito (there were several) and let the system weed them out. If they didn't graduate, they couldn't get someone killed. Not Obito, though. Kakashi couldn't stop his rolling eyes, his disdainful snorts. He kept seeing Obito's flaws, so clearly they glowed like safety-orange in the forest. It didn't matter that Obito would look his way sometimes, clearly stung. No, it just brought it out of him all the more.
When they were assigned to the same team, Kakashi couldn't stop complaining. He was pushed to subversive, unprofessional comments, cutting Obito down in front of the team, and gloating (gloating!) when he succeeded and Obito failed. It wasn't to impress Rin or Minato-sensei. It wasn't to impress anyone, really. It was a matter of doing things the way they were supposed to be done, versus doing them the way a useless crybaby jerkhole might have. He was going to be the consummate ninja. The greatest Hatake clansman, the best squad leader-and titles didn't matter. Respect did.
On some level, all the rookie ninja wanted the same thing Kakashi did. All save for Obito, and so he became an irritating, sentimental, grouchy puzzle that Kakashi's tenacious brain just wouldn't let go of. Couldn't let go of.
Of course, as the story goes, Kakashi did finally see Obito's worth. He finally saw the person Obito was, and the man he was becoming. All it took was his death.
If he thought about Obito often when he was living, it was nothing compared to after that fateful mission at Kannabi Bridge. Obito had become a part of him (in the most literal sense; a not-quite-self fullness in his left eye socket that cried when the rest of Kakashi was keeping it together, if only barely). Obito was his compass, his conscience, his guide.
And when puberty hit, when he expected to be following the curves of firm kunoichi thighs, or if not, the taut planes of muscle that his fellow male nin were developing, he felt... nothing. He learned what went where, like the other kids. He learned how to be coy, how to be assertive, and how to tease, just like the other kids. He performed, as always, above and beyond expectations, even on missions that required that he use his body as both the killing weapon and the bait itself.
But the only thought that would ever give Hatake Kakashi a moment's pleasure was when he imagined what Obito would be like, if he'd survived. He'd still be taller, surely. Sinewy and strong, a little lighter than Asuma. He'd had a lot of potential when they were young, albeit hidden under the softness of lingering baby fat-but Obito would, like most Uchiha, become handsome in that feline, aristocratic way. But he'd smile-he'd smile and be so much more. There would be real honor there. Real understanding.
So sometimes furtively, after it's gotten so bad that he can't stand it, Kakashi will uncover his left eye, and just stare into a mirror. He'll look into Obito's eye, for hours sometimes, and tell it all the things he wishes now he could have said to a whole, living person. Maybe he'll sneak a hand into his pants, but it's always with his eyes on the mirror, and with his mind in the past.
And he will say Sorry, sorry, please... please come back... Like it means something, and I can't do this alone, Obito, I need you here, like someone can hear. And his own, plain dark eye will begin to cry. And Obito's eye will watch him steadily, throbbing, and whirling in time with Kakashi's racing pulse.
When he does this, it always ends for him in tears and regret. He'll bow his head and cry and cry, cry himself sick sometimes. Then, always, he will wash his face, throw his shoulders back, and step out into his world of violence and death, ready to be the perfect assassin again.
But for hours after, his Sharingan-Obito's Sharingan-will flicker images at him, even under cloth covering. He'll see dark walls of rock, and starless midnight skies.
Kakashi doesn't know what they mean, and he doesn't pay them any mind. Obito's not here, and he must simply find a way to survive a world without him.