So, in Sherlock’s mind, everything before this “sets the scene” and this is where we are when its time to start solving the question of Moriarty’s return. Mary has come to make a last ditch effort at reconciliation with her errant husband and Sherlock is trying to keep playing their wedding waltz (the one HE WROTE) while they fight, neither of them able to see what the use of the other one is. John has no idea of what Mary can do, because he doesn’t really see her, and Mary sees John as a continually surprised sidekick to Sherlock, not as his equal. Sherlock gives up trying to play out his version of their romance as soon as Mary starts disparaging John’s role in his life.
And with that, the stage is set. The curtain rises. He is ready to begin. Ready to go deep.