Mycroft is insecure
A beautiful but sad moment in TEH is when Mrs Hudson interrupts the silly conversation of the Holmes brothers and says:
“He’s secretly pleased to see you, underneath all that.”
I think that in this scene Mrs Hudson outwits both brothers again and again.
And after that last spot-on observation, Sherlock looks at her lowkey impressed and remains silent.
This means that Sherlock sees the (double) truth in her words and doesn’t deny it.
On the other hand, Mycroft asks her with a sudden mood shift for the better : “Sorry, which of us?” to which Mrs Hudson replies: “Both of you.”
This is fantastic physical acting by Gatiss here. Mycroft tries to look indifferent and above all that, but he actually feels contentment and great curiosity.
The fact that he, Mycroft Holmes, fails to instantly understand which one of the two Mrs Hudson meant and can’t keep himself from asking an almost childish question tells us that his insecurity about Sherlock’s feelings clouds his judgement.
The sad thing is that, unlike Sherlock, Mycroft doesn’t know whether his young brother actually cares about him like he does. Despite all the hostility, Sherlock knows deep inside that Mycroft loves and looks after him.
When Mrs Hudson responds “Both of you”, Sherlock turns to look at Mycroft playfully, but it is loving and affectionate. He looks at him straight in the eye, which means that he’s finally eager to become more open with his older brother and he would like it if Mycroft did this too. He’s like “See, the goldfish reads us better than we can read each other, brother mine”.
Mycroft may be able to see through Sherlock very easily, but he can’t do it when he’s personally involved. He is afraid that Sherlock truly loathes him. He fears that this is not just a childish sibling rivalry or that Sherlock never forgave him for some deed of the past. He’s very insecure and yearns for some sort of brotherly affection, so he literally clings to whatever information Mrs Hudson might give him. Does Sherlock care about him? Is the “seemingly insignificant” landlady right? Is he loved?!
Mycroft needs Sherlock just as Sherlock needs him, if not more.
I require a hug.