Warning: This post contains spoilers for Game of Thrones.
Jon has always been “the guy who jumps on the grenade to save the rest of the platoon” — as showrunner David Benioff put in the post-episode rundown. However, there may have been another reason he didn’t climb aboard Drogon.
In George R.R. Martin’s A Song of Ice and Fire Series, Daenerys mentions that no Targaryen has ever ridden more than one dragon — a detail pointed out by Reddit user No_conspicuity. “All I know of dragons is what my brother told me when I was a girl, and some I read in books,” Dany says in A Dance With Dragons. “But it is said that even Aegon the Conqueror never dared mount Vhagar or Meraxes, nor did his sisters ride Balerion the Black Dread. Dragons live longer than men, some for hundreds of years, so Balerion had other riders after Aegon died … but no rider ever flew two dragons.”
If Jon is meant to ride Rhaegal, then he may have subconsciously sensed that it wasn’t right for him to mount Drogon. This revelation would also lend credence to the popular fan theory that Jon is one of the prophesied figures known as the three heads of the dragon, i.e., one of three people destined to ride Daenerys’ dragons and save Westeros from ruin.
The three heads of the dragon prophecy comes from a book scene in which Dany has a vision of her brother Rhaegar in the House of the Undying. Show watchers saw this sequence back in season two, but that particular vision was not included.
Before the events of season seven episode six, some believed Tyrion would turn out to be the third rider. However, now that Viserion has been killed and reanimated under the control of the Night King, it seems clear the prophecy was at least somewhat misinterpreted.