Naruto: shifting
What it boils down to is that Sasuke and Kakashi are both lonely bastards who really don't have anything much in common with each other than that whole misery attracts company thing that Kakashi hadn't believed in until now. Neither of them have family. The people who could have been Kakashi's family have been dead for some time now, and Sasuke's remaining family is a murderer who Sasuke wants to become a murderer for. Both of them are sort of iffy when it comes down to dealing with other people and would have failed an interpersonal relations class if forced to take one. Both of them are difficult people. None of this makes for good material to build an attraction on.
The thing is Kakashi has been alone so long he didn't think much of being alone at all, and Sasuke's been alone for long enough so that it hurt and not long enough so that he could brush it off, like Kakashi can. So it's really Sasuke that Kakashi would like to blame everything on, except Sasuke, for all purposes, is still a kid, but not really. And it's technically in some way or fashion Kakashi's fault for letting it go that far, because somewhere in there he should have told Sasuke to stop, but then don't we all realize at one point that you're afraid of being alone for too long. Kakashi has gotten used to it but no one really gets used to it. It's all a lie in one form or another.
It gets out of hand when Kakashi lets Sasuke stay with him overnight. Not because of anything they do, but when push comes to shove and then even more shoving breaks down into placid nonchalant glaring, Kakashi is a traditional man, and traditionally having anyone stay under the covers with you for a night implies some kind of solid relationship, one night stands excluded. But Kakashi knows how it is between Sasuke and him. Not completely unstable, but not completely there, one of those things that could mean nothing in a few weeks, and Kakashi could be dead, and any moment Sasuke could decide that what he really needs to do is fuck all of this childish shit and go find his brother. That's the truth.
Which is not to say that Kakashi isn't glad that Sasuke's here, because he is. He has a vague impression he always will be. It's interesting and crowded in his apartment when Sasuke is over. The little corners of the room seem to brighter, and Sasuke evidently likes more lights on than Kakashi.
As it happens, Kakashi wakes up once during the night because it's warmer than it usually is with Sasuke in bed with him, and then he wakes up again during the night because Sasuke's not under the covers anymore. Sasuke's pulled up a chair beside the bed and is sitting in it, and after a while Kakashi turns on the light irritatedly and asks, "What's wrong?"
Sasuke tucks one knee under his chin and says, "You're not used to sleeping with someone else," and Kakashi is forced to think about how Sasuke phrased that. He turns off the light and turns away from Sasuke, but he can feel Sasuke staring at him still. He wants to tell Sasuke that neither of them are going to fall asleep like that and tomorrow morning they're both going to miserable, but then he realizes that tomorrow morning-- in fact, for many mornings after tomorrow-- they're both going to be miserable deep down inside anyway, and it won't matter how little they sleep tonight.