Mycroft had always warned Sherlock that feelings can get in the way, especially when it comes to his chosen line of work. Typically, this would be the one thing the two brothers could agree on, however, when you reappear on his doorstep after years of no contact, informing Sherlock that your life's in danger, he's left struggling with both his feelings and what happened in your shared past. How can he possibly pretend such strong emotions have never existed when they still remain?