Hey, remember when John got married and didn’t see Sherlock for a month afterwards, and he was really angry and moody about it, and he really wanted to punch something, and then he found Sherlock in a drug den, high off his rocker?
What a beautifully platonic spiral into the deepest throes of heartbreak and depression.
This is a central theme in TAB: Sherlock imagining John choosing Mary over himself. There are at least three instances in which this happens, showing how Sherlock tries to come to terms with what happened in series 3 - pain, heartbreak, loss, near-death. But first there is this because they never shy away from a chance to break our hearts:
HOLMES: You have recently married a man of a seemingly kindly disposition who has now abandoned you for an unsavoury companion of dubious morals. You have come to this agency as a last resort in the hope that reconciliation may still be possible.
This is Sherlock imagining John abandoning Mary for him. At this point he had not even started to go deep into himself and yet we get this wish. He knows perfectly well that this is what he wants even while trying to push John and Mary back together.
LESTRADE: Didn’t Doctor Watson move out a few months ago? HOLMES: He did, didn’t he? Who have I been talking to all this time? LESTRADE: Well, speaking on behalf of the impossibly imbecilic Scotland Yard, that chair is definitely empty.
This scene combines several elements: John moving out of Baker Street (after TRF). Sherlock talking to/looking at his empty chair (TSoT). Sherlock talking to John when he is not there (ASiB).
JOHN: I’m taking Mary home. MARY (instantly): You’re what? JOHN: Mary’s taking me home.
I discussed this scene in more detail here X. This is John leaving Sherlock in the cemetery (to his work) and going away with Mary to his new home.
WATSON (watching the procession): I thought I was losing you. I thought perhaps we were neglecting each other. HOLMES: Well, you’re the one who moved out. WATSON: I was talking to Mary.
We see Sherlock allowing himself a moment of hope, full well knowing that Watson is not talking about him. And, more important, this is one of the most underestimated sentences in the show: John has not just moved out of Victorian MP Baker Street, he has moved out of real 221B and has not come back, not even after Sherlock gave him his miracle.
So there are three rejections, John choosing Mary over Sherlock three times. But this is not the end. There is hope. Because in the the end we get this. This is the true arc of TAB and the show - John choosing Sherlock, John choosing Mary, John choosing Sherlock.