Warning: massive spoilers for the Game of Thrones finale ahead!
Game of Thrones came to an end that was both hopeful and tragic. In the latter category, sadly, are Daenerys and Jon, the final pair of star-crossed lovers whose loyalties and bloodlines meant they could never have a happily ever after together. In the end, it's Jon who kills Dany, professing his love and loyalty just before stabbing her to prevent her from continuing a reign of violence and terror. Before he does so, though, he has a lengthy conversation with Tyrion Lannister, trying to sort out his complicated feelings about Dany.
"I know you love her. I love her, too. Not as successfully as you, but I believed in her with all my heart. Love is more powerful than reason. We all know that," Tyrion tells Jon. In response, Jon bitterly quotes Maester Aemon from years ago: "Love is the death of duty." It's a quote that was told to Jon long ago, explaining why the Night's Watch are forbidden from having lovers or families: because their duty must come first, but love always ends up overriding one's duty. Jon already broke that rule several times over, but now he's forced to do it again.
It's a significant callback, especially given that Jon ultimately does his duty (protecting the kingdom) rather than following his heart and staying with the woman he loves. And yet, it's a different sort of love that drives him to do so — after all, he doesn't entertain the thought of killing Dany until Tyrion mentions that Jon's sisters won't be safe if Dany stays on the throne. Love and duty have so often been at war throughout Game of Thrones; it's sad and fitting that a decision between the two is a final note in the series.