There may be better soldiers in Game Of Thrones, there may even be better people, but there is probably no better friend in Game of Thrones than Samwell Tarly. Who better to be the recipient of Jon Snow's reaction to being a Targaryen than his best friend, who did some of the legwork to discover this crucial fact? As Samwell explained to him in the crypts beneath Winterfell, "Your mother was Lyanna Stark. Your father — your REAL father —was Rhaegar Targaryen. You've never been a bastard. You are Aegon Targaryen, heir to the Iron Throne ... You're the true King. Aegon Targaryen, Sixth of His Name, Protector of the Realm ... all of it."
Jon's reaction brought out a lot of complicated emotions and it seems the matter of his parentage could affect his budding relationship with the Mother of Dragons, who he just recently bent the knee to in more ways than one. Samwell, like many best friends, isn't a huge fan of Jon's new girlfriend. While most guys don't like it when their boys start dating someone new because they start having less hang time, Samwell seems to take an issue with the fact that Jon's Queen put Sam's father and brother in front of a Dragon and then told it to breathe.
Sam didn't have the best relationship with his father, but is rightfully still pretty bummed out that his bestie's partner violently murdered his own family to prove a point and then told him such in an extremely matter-of-fact way.
This may be why Jon tried so hard to poke holes in Sam's declaration that Jon is the rightful heir to the Iron Throne — well, that and the fact that if Sam is right then Jon is definitely in love with and has had sex with his aunt — but his reaction to the idea of being King suggests that he'll have a lot to think about should he survive the marching wights.
Jon Snow has never been a fan of power, attempting to push it away every time it is thrust upon him. And yet, when Jon Snow assumes such power, he takes to it like a Lannister to incest, calling orders and insisting that he knows the best action to take in any given situation. The title of King of the Seven Kingdoms would finally place him in power, and will probably be the only way he'll ever be able to feel truly safe in Westeros. It also means that he would have to rob his family of the opportunity to sit upon the Iron Throne, as well as the love of his life because they are the same person.
Jon Snow's life has been full of twists and turns, but the fact that his father is not Ned Stark may be the biggest yet. Much of Jon's identity has been wrapped up in being the bastard son of an honorable man, making his tale an underdog story of sorts — the Rocky of Westeros. However, now that Jon is officially highborn and his aunt-lover is his competition for the throne, the Song of Ice and Fire could turn from a love song into one of heartbreak and anger.