Because I can never have only one idea.
Yamato takes the position of Orochimaru’s watcher not only because he feels responsible, also because he wants to run away from Kakashi.
After the war, while Yamato is struggling and feeling weak for his capture during the war, he notices how Kakashi spends less time at the grave and takes more time to stay by his side. And then Yamato recognizes the love in Kakashi’s tender eyes. Just like his Mugen Tsukuyomi dream. A dream is a dream because you can never have it. Yamato is afraid that everything is only a dream like he imagined once he accepts. So he runs.
Kakashi understands how painful it must be for Yamato, and he wants to help. It’s sad when Yamato not accepting, and Kakashi blames himself. He knows for years, Yamato, since he’s Tenzou, did everything for him out of respect and love. But he never returned them. He didn’t want to get attached; it always hurts too much when you lose an important one. So Kakashi pushed those emotions aside, taking whatever affection he could get from Yamato. Kakashi also knows how to give small praises to make Yamato happy enough, like giving treats to a dog. But Yamato is more than that. The feel of a growing empty hole in his chest after hearing report of Yamato being captured, Kakashi knows how much he didn’t want to lose Yamato. He would do anything to help Yamato, even if it means that would take years for Yamato to accept.
Yamato cowardly takes Kakashi’s offer of treats when he’s back in the village for reports. They go out to eat and walk around in the village chatting, almost like a couple. Almost. Till one time when they are having drinks at Kakashi’s place, Yamato having a bit more than he should. Yamato is half-asleep and laying his head on Kakashi’s shoulder, while Kakashi runs his hand on Yamato’s back smoothing him, sometimes playing with his hair. Then Yamato hears Kakashi’s low voice. “Tenzou, will you let me love you?” Yamato has never been loved. He doesn’t know how’s that supposed to work and if he’s capable to accept the love from the man he has admired for so long. He pretends to be sleeping, but his mind is crying out loud. Maybe also a little in real tears.
Then Yamato starts taking longer time at his duty than coming home, to avoid Kakashi. Kakashi doesn’t say anything. He just waits, patiently. He had let Yamato wait long enough, and now it’s his turn.
(Because everyone’s favorite 7th Hokage gets all annoyed at his former teacher after knowing their former team captain is still working more than he should. He sends a team to cover Yamato’s job and has Yamato back in the village.)
When Yamato finishes his guard duty and returns to the village, he sees Kakashi waiting for him at the gate. Kakashi gives him a smile and offers his hand to Yamato, asking if Yamato wants to return to their home. Yamato realizes how similar everything looks like in his dream, except they’re much older now. Kakashi smiles gently, and Yamato sends a smile in return. Unlike in the dream, Yamato doesn’t give a wide grin, but a nod and a shy smile, then taking Kakashi’s hand. They share their walk home holding each other’s hands. When they arrive and Kakashi welcomes him home, Yamato replies. “I’m home.”
(The place they share now is built on the Hatake property Kakashi inherited. Kakashi wanted to resurrect his home ever since the village got wiped out, but he lacked the inspiration for years. During the time of Yamato’s absent, Kakashi slowly develops his dream home with sharing it with Yamato in mind. After it’s built, Kakashi tries his hardest to convince Yamato to move in with him. At some point he just accuses Yamato wasting money on rent because he doesn’t live at his place most of the time, though Yamato only jokingly replies if Kakahsi cares so much then he should pay. Kakashi is indeed serious and pays the rent for Yamato. A few months after, Yamato gives in and moves into Kakashi’s new home. The house is surrounded by trees, quiet and beautiful. Yamato’s room is filled with sun light. Everytime he comes back from the mission and sees the plants by his windowsill growing well, he smiles, knowing Kakashi spends time to take good care of them. And when he hears a knock on the door and sees Kakashi standing at the door smiling at him, Yamato feels maybe this is how being loved works.)
(It’s after a few days, then Yamato knows that since Kakashi hearing he’s coming back to the village, he’s been waiting everyday at the gate for Yamato to return.)
