I think that naruto's dumbness comes from the burdens he got in the past:
- Lonelyness and hate in his childhood
- Roguetrip of Sasuke
In his childhood, he tried to compremise the hate and lonelyness by being a dumbass to get some attention.
After Sasuke left the village, naruto became vurnable about anything that had to do with Sasuke. This got even worse when Sakura cried like a **. If something was involved with the Sasuke case, Naruto rushed like a dumbass.
After he got acknowladged by almost everyone, Konoha, Sandvillage, (the zabuza episode village) he got freed from the lonelyness.
After he talked with Sasuke, he got loose from the case. He knew what he had to do and didn't think alot about it anymore.
And now he got freed of alot burdens and can think clearly and is being an excellent ninja now.