Naruto Shippuuden Episode 46

[KrayZ33]

well another reason for his faceless expression could be that he lost his "faith"...

you see how energic he looked when he fought his enemys? his eyes were angry and so was his mouth (it was drawn simply but its more easy to figure out his feelings) and then, after reaching the middle, he realized that everything he just did was pointless.. his brother doesn't come and you know, we cant be sure if he even died for real... what if his brother just stopped fighting and has forsaken sai? (so if sai says "my brother is already dead" it could mean that he has forsaken his brother too... he is pretty cold hearted now, which could be the consequence of this.. its like "i've fought with all my heart to reach my dream but it was useless in the end, so why should i even bother now?")

that would explain why he is still carrying that book around... he hopes for his brother to come, so he can finish the book.. but in reality he knows that he won't... BUT when his brothers will make it somehow, he can draw a smiling face if he wishes to... so its like to have an ace up one's sleeve

he wouldn't have left the other site blank otherwise.. because if he is certain about his brothers death then he would have drawn a site with a background only i think.
that would express that someone is "missing" because in the point of view of the reader the last picture isn't focussed on sai but on the middle, which leads to the thinking of "something missing" for the viewer