When he met Naruto for the first time, Sasuke was hiding behind his mother.
Sasuke was staring suspiciously at the goldy-head who was giving him an eyeless grin; Naruto's smile was so big it extended up above even his eyebrows and made his eyes rise upwards into two cheerful arches. Sasuke's mouth stayed in a straight line, contemplating whether to accept or refuse the blond's obvious invitation to become friends.
The thoughtful concentration on Sasuke's baby face was so funny that had not his father been present, Uchiha Itachi would've laughed out loud. It amused the teenager to no end that such a simple decision could complicate and entangle itself so much in Sasuke's miniature brain.
Getting impatient, Naruto thrust out a hand for Sasuke to shake, startling the dark haired boy out of his deep thoughts.
"HI! I'm Uzumaki Naruto. I'm five and I like ramen. Let's be friends!"
Sasuke hesitantly held out his own hand, which Naruto grabbed and shook up and down vigorously. He hadn't made up his mind yet, but to have rejected the blond's hand would have been impolite, and Sasuke was raised better than that.
Naruto then proceeded to proclaim to his father, the Yondaime Hokage, that he and ("Uh, I forgot to ask what your name was. Oh okay.") Uchiha Sasuke were now buddies.
"Look 'tou-san a new friend!" Naruto exclaimed gleefully, then grabbed Sasuke's hand again and ran, dragging the older boy behind him and yelling over his shoulder: "Me and Sasuke are going to play in the park 'tou-san!"
Thus, at the tender age of five, Uchiha Sasuke unknowingly sent his sanity to the grave.