When John Watson is left alone, he's at a loss of what he didn't say, what he didn't realise before it happened.
The Fall.
It ripped John open, he felt raw in those days, weeks, months, years, after it happened. He still sees the face of his best friend when he closes his eyes to sleep, the billowing coat. The blood. He never knew what he had until it was gone, and now it's gone, he wants it back more than anything. 221B isn't the same without the melodious notes of the violin, the raucous gun shots and the sound of cries of joy at the knowledge of a murder. But now, more alone than ever, John doesn't know how to cope without him. He considers moving, getting away from London all together, but every time that he has gone to stay with Harry, he feels further away from reality than ever before, he can't get away from London and the place where it happened. Where he died.
Where Sherlock Holmes left him.