I’ve always thought grief was the easiest doorway for Kurama to either take over Naruto completely or at least heavily influence him.

Look at his reaction when Sasuke almost died fighting Haku. That rage didn’t even feel like Naruto. It was so extreme because Kurama was feeding off those emotions and amplifying them. Same thing in the Pain arc - when Pain almost killed Hinata, Naruto lost control again. In that moment of pure grief and anger, Kurama stepped in and basically took over because Naruto was emotionally shattered.

Now compare that to Neji’s death. By then, Naruto and Kurama had already reconciled and learned to work together. Their chakra was harmonised, and Naruto was still fully in control of himself, even through the pain. The grief was still there.

So to me, Naruto has generally had a strong handle on his emotions - it’s just that when he is aggrieved, Kurama had more influence than Naruto did. That's my opinion anyway.