Because it's perfect, that's why. ok I don't ship Johnlock but this is epic. <-- I'm not gonna take this as Johnlock but instead as uncle John reading to Sherlocks and Molly's son Hamish<<how can you not ship Johnlock? This is Johnlock!
And when Molly tried to imply he only cared about John's opinions/concerns he quickly let her know she always counted too. So all we've really learned from this is, when pressed, he will be considerate of peoples emotions.
Mark, you're gay so you won't kiss me, but you are a brilliant writer. Even though your show pains me to the very core, I love it with every fiber of my being (*waiting for a kiss on the cheek from my favorite writer*)