When Sherlock had decided to fake his own death, he hadn't considered all the mourning on the behalf of John Watson.
He came to the graveyard every week at first, and then once a month, and then maybe once in a while.
But when he did come, he stayed for hours. Just... talking.
Sherlock heard him. He heard him, and it broke his heart. He had "committed suicide" in order to save John's life, but from what he could see, he was destroying it at the same time. And Molly had made that very clear.
"SHERLOCK YOU INSOLENT PRICK."
Yeah, Sherlock could still feel the slap he had received. He hadn't planned on waiting this long, (two years had felt like 20 when you're pretending to be dead,) but every day it became harder to come back.
And then the feelings had started.
Sherlock had always loved John, but it was bearable to remain quiet about it when he was around John. But now...
So Sherlock just stood in the background, watching the love if his life move on with his life.
Sherlock should be happy that John was no longer languishing after the detective'a death.
But human error was a nasty thing and Sherlock couldn't help but feel he was lost forever.