As a children, Mycroft used to tell Sherlock stories of magic and stuff. Sherlock used to love the stories, but he was obsessed with the Dragonborn Mycroft told him about.

As he grew up, he would read up on the magic, shouting, and the origins surrounding these stories, and continued to do so into adulthood, even though he grew out of daydreaming about it all.

Then he meets John. Sherlock knows there's something special about him, but he can't quite figure out what.

(tl;dr)Turns out John is a dragonborn, and Sherlock thinks it's awesome.

Up to the author how it's revealed and all that.