There was a lot of pressure on Itachi. He was the prodigy of the Uchicha and the next head of the clan. He was also their spy in ANBU (but not really, because actually his first allegiance was to the Hokage). Most thirteen-year-olds would have cracked under all that pressure, but since Itachi was Just That Awesome, he was still mostly sane… up to a point.
It wasn't really any surprise when Fugaku told the Council of Elders that Itachi would not be around for a good while, thank you very much. It happened sort of like this…
Itachi had been sitting in the same position in the middle of the private Uchicha training grounds for an hour now, and Sasuke really needed to practice his shuriken throwing. He walked over to his brother and tapped the older boy on his shoulder. "Um, Itachi-niisan?"
The prodigy leapt up suddenly and assumed a heroic pose, his ANBU-issued katana in his hand.
"… Itachi-niisan?"
"I am Don Itachi, the knight errant of Konoha!" Itachi proclaimed proudly. "I, Don Itachi, proudly swear to uphold the laws of the Leaf Village for ever! Come, Sasuke, my faithful squire, let us be off!" The boy looked around, seeing a conspicuous absence of his little brother.
Sasuke, just out of view behind the door that separated the training grounds from their house, tugged on his father's sleeve. "Papa, Itachi-niisan is…"
Fugaku sighed, looking out of the window at Itachi, who seemed to be shouting something extremely loudly at a passerby. "I know, son, I know."