Sasuke doesn't know what made him do it. It was stupid and reckless and he deserves the lashes of the whip on his back.
He thinks it's okay though. The look on Naruto's face said he hadn't forgotten him.

Naruto doesn't know why Sasuke did it. It had shocked him and left him unable to move, unable to chase him.
He thinks it's okay though. The look on Sasuke's face said he would come back someday.

Neither of them know why Sasuke came back to wish Naruto happy birthday. Both of them know that it means everything will be okay.