Why did Drogon allow Jon Snow to pet him in Game of Thrones S07E06? What could be the possible reasons for that when the dragons only listen to Daenerys Targaryen.
As elaborated in this article:
The best idea for why Dragon seems to like Jon Snow is because of Jon’s Targaryen blood. As we learned last season, Jon is not Ned Stark’s bastard child, but is actually the child of Prince Rhaegar (Dany’s older brother) and Lyanna Stark, Ned’s sister.
The Targaryen in Jon Snow’s blood is likely making Drogon more amiable to him. Daenerys thinks she’s the last Targaryen left, which makes Drogon's reaction to Jon even more surprising.
Book readers know that not every one of Targaryen blood is automatically friends with all dragons. But it’s unclear if that’s the case in the show.