Despite the entire episode existing, I think this is one of the most sobering moments in the show. Sherlock getting ready for his arrest.
He’s clearly putting on his armor in preparation for the final battle of the war that has been raging between him and Moriarty. He knows there a good chance that something very bad is about to happen to him. So he’s allowing himself to be taken away from the things he cares about, from Baker Street, from Mrs. Hudson, from John.
But then John—stupid, mad, brilliant, ridiculous John—has to go and chin the Chief Superintendent. Because John knows when someone is gearing up for battle and he’ll be damned if he let’s Sherlock walk into it alone.