Actually Sasuke had to depend on the seal late in his fight with Gaara in order to use the Chidori a third (4th?) time; even with the curse seal he couldn't beat Gaara and was going to sacrifice himself to protect Naruto and Sakura ...

And Then Naruto went SSJ4 on Gaara and Sasuke flipped out.

So - Sasuke was upset that Naruto had been way better than he expected (to sasuke's view this growth had come out of nowhere - he had missed the end of the bridge fight and did not know about Naruto defending Iruka-sensei in the early manga).

Then a Month later Naruto busts out the Rasengan, and Itachi ignores Sasuke, while persuing Naruto...

Naruto had not only seemingly surpassed his strength - Naruto and his sensei (Jiraya) were fighting Itachi and seemingly holding thier own...

Sasuke was in danger of watching Naruto defeat his brother and render Sasuke's life COMPLETELY POINTLESS..

Sasuke had to have power NOW.. he is obsessed with becoming more powerfull than Naruto (and Itachi) IMMEDIATELY... but he doesn't understand what he's gotten himself into - his decisions have all been paniced ones.