This sequence bears more attention. Vatican cameos means someone is going to die. It’s a warning to John of impending disaster to a life. John acts as his conductor of light bringing the dots that Sherlock joins up to solve a case into clear light “You keep me right” telling John that it is always John who performs this important role for him. “always you” nobody else makes his brain work better and faster latching on to the relevant dots that make up the deduction big picture. He tried solving cases with Molly but there isn’t that same ‘chemistry’ that igniting spark that lights his deductive flame. He knows that it must always be John. He doesn’t need an audience for his genius, there’s only one person he needs and who he wants to work with, that’s John.
“What do I do?” John is ready for his orders, the team of two against the rest of the world, A life matters more than his wedding, his ‘special day’ the ‘most important’ remember when he asked Sherlock to be his Best Man? The call to arms from Sherlock is all John needs to step up to the plate.
What is important here to Sherlock now, though, is not solving a murder but to “Save the life” preventing a murder and he’s just told John his importance in he scheme of things, how John is important to him. “It’s always you” and they are going to save another life together. Look how ecstatic Sherlock is, how he admires John as he walks, ignited.