Naruto is the first one of them to stop calling him sensei.
It hurts a little, because he always thought, if nothing else, Naruto would always be immature enough to need a sensei. Someone to look after him, guide him, keep him alive when his temper gets out of hand. But after his training, after he finally masters Rasengan, Naruto turns away from him. He's done; he has what he needed. Kakashi is no longer relevant to him, not in a serious way.
It hurts, but he understands. The boy is always polite when they cross paths, but there is a look in his eyes; a look of anger, of stubberness, of hatred. Not a lot - Naruto is too kind-hearted to ever truly hate him - but his memories run deep, and everytime Naruto looks at him, Kakashi can see the memories run behind his eyes - all the times he complimented Sasuke (and Sakura, to a lesser extent) and not him; all the times he ignored Naruto (and, again, Sakura) in favor of his genuis student; all the times Naruto needed him to simply be there and he wasn't.
None of this shows on his face, of course; Naruto is (and always will be) a fine actor - he has to be, in order to survive in Konoha - and he always has a smile for his former team leader, whenever they see each other. But he will never forget, even if he has forgiven - some things stay forever, no matter how hard you try to remove them.
He wants to take him aside, sometimes, and explain himself. He wasn't trying to ignore him - like so many others do - in favor of someone else, wasn't trying to hurt him. But he was, and he did, and he knows it. He wants to explain that he's human, and he can't outrun his past, still can't lay his ghosts to rest, still can't forget them, forget him.
But Naruto has already turned away from him, just like Sasuke, and Kakashi knows that the time has long since passed.p
But it still hurts, and he thinks it always will.