Like every episode in Season 8, the Game of Thrones finale had its share of callbacks and references to the journey many of the characters undertook to reach the story's end. A big one in Episode 6, "The Iron Throne," popped up as Tyrion Lannister and Jon Snow talked about what to do about Daenerys Targaryen in the wake of the destruction of King's Landing. The discussion recalled something that came up at a critical moment way back in Season 1--a moment that had a great deal of meaning for Jon. Be warned: We're talking spoilers for the Game of Thrones series finale here, so read on at your own risk.
After the sack of King's Landing in Episode 5, "The Bells," Daenerys has Tyrion arrested for freeing his brother Jaime, going against the queen's orders and thereby betraying her. Jon meets with Tyrion in his impromptu cell, where Tyrion laments finding out he was wrong about Daenerys. He tries to convince Jon to do something about Dany since Jon is also a Targaryen and has a better claim to the Iron Throne. Jon, of course, struggles with the situation because of his love for Dany, and Tyrion talks about how he understands that Jon loves Dany.
"Love is the death of duty," Jon responds. He's remembering a pivotal moment from his past--a conversation he had with Maester Aemon, way back when Jon was new to the Night's Watch in Season 1. During that discussion, Aemon revealed his true identity as Aemon Targaryen, a man who could have been king, but chose the life of a Maester and a man of the Night's Watch instead.
At the time, Jon had been considering breaking his new Night's Watch vows to ride south and join Robb Stark in his campaign to defeat the Lannisters and rescue Ned Stark. Aemon stopped him by telling him a story about how his own vows were tested during Robert's Rebellion. He wanted to go south to help the other Targaryens, but ultimately, kept to his vows and did his duty. It's that story that convinces Jon to stay on the Wall and keep his vows, as well.
Jon's sense of duty has repeatedly been tested by love. He didn't leave the Wall to aid Robb, which meant that Jon didn't die at the Red Wedding, but one wonders how Jon's presence could have helped save Robb's life. When he went north of the Wall with Lord Commander Mormont, Jon joined the Wildlings he discovered there to find out what Mance Rayder was planning. He fell in love with Ygritte but was unable to betray the Night's Watch for her, which ended in her death in Season 4 at the Battle of Castle Black. And finally, Jon chose duty over Daenerys, killing the queen before she could hurt anyone else after she destroyed King's Landing.
In the end, Jon always chooses duty over love. Tyrion gets him on that point before the conversation is over.
"Sometimes, duty is the death of love," Tyrion says. "You are the shield that guards the realms of men, and you've always tried to do the right thing, no matter the cost. You've always tried to protect people. Who is the greatest threat to the people now?"
Tyrion knows how important Jon's sense of honor is to him, just like Maester Aemon did (although Aemon might have given Jon different advice if he'd known that Aemon was Jon's great-great uncle, or that Jon was intending to kill Aemon's niece). Jon struggles with whether he made the right call in killing Daenerys, but we've seen throughout the show that it was the only one he could have made.