I think the most revealing exchange about Mycroft and Sherlock is at the end of A Scandal in Belgravia, when he says, ‘Sherlock has the brain of a philosopher or a scientist, but he likes to be a detective. What might we deduce about his heart?’
There’s tremendous romance in Sherlock Holmes. He needs to be fighting crime and fighting bad guys. He doesn’t admit that to himself, but there are plenty of opportunities for him to go and think elsewhere … but this is what he chooses to do.
So [Mycroft] doesn’t have that drive for color or glamour. Mycroft just wants to be alone and precise and do all his thinking.
No, he hasn’t allowed anyone to get close to him at all … except he’s quite close to his brother, from the very start — that’s not something that’s changed — from the very, very beginning of A Study in Pink.
[Mycroft’s] trying to look out for him. He’s worried about him. He’s worried about what his junkie little brother might do next, as you would be in real life. If your little brother or something wanted to be a detective—not a junkie anymore, now a detective, because crime is really fascinating—you’d be worried. He’s worried, that’s all.”