The dynamics of Team Seven was always a strange one. They were friends. They were enemies. The were strangers. They were siblings. They were lovers. They were teammates, enemies, parents, rivals, and so many other things. They were bound by pain, and separated by agony. They were all the same, and yet each of them was so different.
"Fate-bound," Sai remarked one day before training. Naruto gave him a weird look and Sakura simply ignored him, figuring that it was just Sai being weird again.
Kakashi arrived a few seconds later. Sai never explained his comment, and the others didn't ask. But every time Sasuke was brought up Sai just smirked internally. Because, really, the definition of Sasuke didn't start with, "teammate," or "friend," or "avenger." His definition was much like Naruto and Sakura's, so complex that only Sai's explanation fit. He just hoped that Sasuke figured that out soon.
A while later they tracked Sasuke down at Orochimaru's hideout. Sasuke hadn't really addressed them at all, and Sai had to think that that meant Sasuke was beginning to figure it out. He couldn't break his bonds with the two because they were bound by unbreakable fate. He spoke of "whims", when, really, Sasuke had looked fate in the face and told it to fuck off. Fate, obviously hadn't liked that, and that is the only logical explanation Sai can think of that explains why Naruto is here with a working, beating heart.
They don't bring him home, but that's okay, because he's bound to them so he will return eventually.
Two months of Sai referring to Team Seven as "fate-bound" and Sakura finally asks.
"According to Neji fate defines everything. Team Seven is everything."
Sakura doesn't respond, but smiles a little. She doesn't bring it up again, but she begins to use the term herself.
It took Naruto another month before he was curious enough to ask. Most of the time it wasn't worth trying to get Sai to explain something.
When Sai told him, Naruto broke out into a large grin, a sparkle of unshed tears obvious.
In a voice much quieter than he has ever used, Naruto thanks Sai. Sai still doesn't understand what he was thanking him for.
It is another heartbreaking long four years before Sasuke shows up to Konoha. Showing up sometime in the night, he waited for his team by their old meeting spot. He was assaulted with hugs, punches, tears, and of course the one question on everyone's mind Why have you come back?
Sai replies before Sasuke gets a chance, "Fate-bound."