"Best Friends Forever"
Even though it came almost as a surprise for Naruto that Sasuke left the village in order for revenge, joining Orochimaru, it wasn't. He had felt an underlining feeling in Sasuke, his best friend, and also a subconscious feeling in himself telling him something, something that was unable to be worded. It could be said that he almost knew that it was going to happen.
But what left him as was said, almost of a surprise, was when he actually left. He felt unprepared for that moment; he was unprepared for his best friend leaving him for revenge.
What could have he done to prepare or stop Sasuke from leaving him? Have a talk with him? Have a discussion? Recommend a councillor? Sasuke would have beaten him up for even thinking of that. Maybe he should have plain said: "I know you want revenge for your clan Sasuke. But in the future, you'll join disgusting Orochimaru to grow stronger. Orochimaru, Sasuke. Orochimaru. And not just that, but you'll be leaving me too…"
Yeah right.
But if he could go to the past and be able to do something, he might consider doing that.
After all, Sasuke is his best friend… his first true friend. His answered wish.
"I wish…"
That was also why he tried so hard to bring him back, why he was brought back with Kakashi carrying severe injuries that were slowly being healed by the Nine-tailed Fox within him. He was drained even with the help of the demon. He was crippled by his emotions, not by his long gone physical damage.
And the promise he made with Sakura. It was left unfulfilled. Almost broken.
What about his way of the ninja?
It hurt him even more when he faced Sakura, Sasuke-less. He knew Sakura was relying on him, and that she was hurt as well.
Kakashi no longer had a team. What emotions could he be feeling?
What about the people left by Sasuke?
He was so hurt. So deeply hurt.
What about the effect of Sasuke leaving… the consequences?
Scars are left and they hurt. Does time really heal everything? Won't he shrivel up and rot in pain over time?
How he wanted his best friend back.
How he will continue to reach out to him and attempt to grab him and get a good hold.
How long will this continue?
Until he shrivels up and dies.