Because he looked at those hateful damned red eyes and he felt nothing. Not a single thing. No rush or flurry of emotions, no fireworks, no butterflies. All he felt was a burning hatred and the aching of his heart.
Because he clenched his fists harder then anything, like he was desperately trying to hold the air in his closed palm. He bared his teeth and crossed his brow and his breathing got heavy in an angry and not a sensual kind of way. The blonde hair on the back of his neck stood up in murderous desire and the whiskers on his cheeks grew and flared in a maddening craving to tear the black haired, red eyed boy's body limb by limb.
Because the only thing he wanted to say was "fuck you" and not actually have the desire to do it. A buzzing was in his ear, irritating his senses and making him even more annoyed, more angered. All he smelt was the blood running in that cold bastard's veins and not the smooth cologne he normally wore before he left. Looking up at that stubborn son of a bitch and seeing the uniform of Orichimaru on him drove him mad and he wanted to rip every piece of clothing off of his body; not in a sexual desire, but in a deeply murderous and hateful want.
Because he tried to forget—so many times—all the things they did together, and all the things they shared. How he cried at night, clutching the only picture they'd taken and desperately pleading to anything that would listen to bring the cold hearted asshole back to him. The promise he made to bring that fucker back, but broke because he wasn't strong enough to keep the bastard from leaving. And the training he'd gone through just to make it to this one moment, this single moment that pictured for the last two and a half years.
Because Naruto looked at Sasuke and all he felt was a burning desire to bash his face in. To break him and make him fall to his knees in forgiveness.
Because Naruto looked at Sasuke and all he remembered was their love. When they held hands and talked about things that didn't matter at night.
Because Naruto looked at Sasuke and all that love, that faith, that dependency, that adoration, the fireworks, the butterflies—all of it was finally and wretchedly gone.
Why?
Because Sasuke left.