Jealousy
He watched as Naruto Uzumaki clenched his fists, declaring that he would become the next Hokage, his cerulean orbs mirroring hope and determination as he continued to gripe in Kakashi's ear, openly making a fool of himself. Like always.
Nonetheless, he kept trying, constantly being shot down but still striving for the love that the other boy had so carelessly tossed aside. Whereas Sasuke, he strived for vengeance in the bitter darkness, something which the blonde, replete with light, couldn't comprehend. He should've shut himself away, like his opposite.
In some ways, Sasuke Uchiha despised that boy, his best friend.