God, this line, this scene kills me.

In a way, it highlights the differences between the Holmes brothers. Mycroft enjoys being above other people and their petty emotions, having more control over himself than them (and more control of them). They are weak because they care. He truly believes “caring is not an advantage,” and so he has no one to care for except for the younger brother that he thinks is like him and the country that he serves. But Sherlock isn’t so convinced. He’s surrounded by people who care about him and who he, despite himself, cares about. He chooses a career that brings him down among the masses where he sees their caring and their emotions every day. He has the mind to set himself above and apart, like Mycroft, but he doesn’t. So he finds himself wondering if he’s wrong, if there’s something wrong with him because he doesn’t care quite the way other people do - but he does care.

And so Mycroft worries, because that caring is a weakness his brother can’t afford. Here, he takes the chance to remind Sherlock that caring is a costly weakness.