Here’s the thing that broke my heart:
In “The Sign of Three,” Sherlock calls Mycroft right before his best man speech, telling him that it’s not too late to attend the wedding, which is ridiculous and out of character for Sherlock, because Mycroft’s right, John and Mary wouldn’t really want Mycroft there. So why did Sherlock call? Mycroft recognizes Sherlock’s anxiety (about the wedding, about the future, about his best man speech, etc.) and so he reminds Sherlock of “Redbeard.” At that point, we didn’t understand what that meant yet. But Sherlock reacted dismissively, telling Mycroft that he continues to infantilize him.
Then, in “His Last Vow,” we find out that in a moment of crisis, as Sherlock is desperately trying to stay alive and going into shock, Sherlock forces himself to seek a calming memory (tellingly, the forcible voice of his mind is Mycroft). And in that moment, he finds Redbeard, revealed to be his childhood dog. His dog comes to him in this moment of crisis, and comforts him as dogs do. Oh my god, my heart.
So, what Sherlock dismisses as an infantilizing, patronizing mention by Mycroft was actually his brother’s attempt to calm Sherlock as he uncharacteristically reaches out to Mycroft in a moment of anxiety at the wedding.
However, I don’t think Sherlock will really see Mycroft as someone who cares for him deeply, as their relationship is such that Sherlock always seems to think the worst of him.