Spoiler warning: This covered the aftermath of Naruto and Sasuke's showdown.
No Devotion Like Love Betrayed
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The biggest problem with Sasuke's obsession with Itachi was, in Naruto's opinion, the fact that he admitted it.
Had he denied it, shown disapproval towards any signs of obsession, Naruto would just have to shove it to his face and force him to see it. Sasuke was reasonable enough to accept proofs, disciplined enough to put an end to a bad trait.
But Sasuke thoroughly accepted the obsession as the way it was and should be, totally accepted Naruto's reproach over it. He had no intention of changing, nor felt the need to.
The second biggest problem was how he dealt with it. Naruto was still fuming over why the hell he was the one Sasuke insisted on a death match against, when clearly Itachi was the source of the problem. Why should Naruto's friendship be the one to suffer (not even Sasuke's, since clearly their friendship didn't mean anything to him) for the sake of someone he didn't even know before?
That led to the third biggest problem, which was at the third place only because Naruto didn't quite like admitting it himself. Even if friendship turned out to be an illusion on Naruto's part, Sasuke had been a great teammate, a rival in a good way. Then all of a sudden he seriously attempted to kill Naruto, and when he failed he just went off without an apology or a goodbye; not even a promise to finish the fight one day.
The third biggest problem was apparently that Naruto was obsessed with Sasuke just as much. And it hurt, not only because Sasuke put Itachi above him, but also because Sasuke didn't care enough not to trample on Naruto's feelings in the process.
The biggest problem with Naruto's obsession with Sasuke was that it was deep enough for him to understand Sasuke, to forgive the things Sasuke did because of Itachi... and yet, enough never to let go of what he wished it could've been between them.
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