Naruto was mad and full of rage before Hinata arrived and could have busted Kyubi mode at any time. He was trying hard to avoid that option. Before Hinata arrived Naruto was piled on with emotion-sadness, anger, frustration, shock, rage, helplessness. When Hinata comes along Naruto is clearly emotionally weakened and helpless. Had Naruto been emotionally strong when Hinata "died" Naruto may not have bursted into Six-Tailed Kyubi.
What I'm saying is that Hinata was the last straw that broke the camel's back. There's no need for a confession to make her sacrifice important. You could remove the confession and it would still have the same effect.
Prehaps I'm poorly explaining what I mean. xd
She's the only one who would've said the things that she said. And you can tell by the look in his eyes that her words have an effect on him :B A friend falling in battle would've been different :U
There doesn't have to be a confession. While Hinata's words had Naruto widen his eyes (the confession was unexpected), there was nothing to show it was her love for him that was important. It was her getting hurt.
We have a flashback of her dying and one of her jumping in to save him. Yet there's no panel of him remembering her words. Not to say he doesn't remember, but he
may not remember. It may not have sunked in fully yet.
If it was just a friend getting hurt, then KN6 would've happened sooner. Mind you, I'm not saying he's in love now or anything ^^; Just that she's become more important to him than she was before.
It's because it's Hinata. See, if Kishimoto had of had Naruto bust Kyubi when Fukasaku was killed or had of replaced Hinata with say Ino (minus confession) the fans would be pissed. Save for NaruIno fans.
No matter how much it makes sense for Naruto to go Kyubi for any of the Rookies (as they're his friends) the author will choose the most popular option.
There's nothing to show it was Hinata's confession of love that triggered his rage. Naruto wanted acknowledgement. To be noticed. Nothing in the manga shows Hinata being more important just because she said she loved him.