Kakashi, Naruto, and walking away...
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/,
But Naruto has already turned away from him, /just like Sasuke/, and
Kakashi knows that the time has long since passed.
But it still hurts, and he thinks it always will.