I bet Sai is really strong and then Naruto gets mad because he's showing off too much. Though Sakura didn't like Sai at the start, she starts to like him and Sai starts to like her too. This makes Naruto really jealous and he does his usual stuff when he gets jealous. It will all be similar to when they were young and Sasuke was still there, but the only difference is Sai likes Sakura. Or maybe he plays with her feelings which gets Naruto pissed also!

I dunno... It's just a theory =D