Jon Snow's been through quite a journey on Game of Thrones, starting as the Bastard Son of Winterfell before becoming the 998th Lord Commander of the Night's Watch. In Season 8, Jon Snow's Title is "King In The North". That could change by the end of the series if he ends up on the Iron Throne, but for the time being he seems perfectly happy being the King In The North and bending the knee to the Mother of Dragons.
Although he grew up with the Starks, he was always called a bastard, even if he did get his very own direwolf, Ghost, to bond with. Although he spent most of the Game Of Thrones series on his way to, beyond, or back from, The Wall, it wasn't until after Melisandre resurrected him that he decided to make his way back to Winterfell and call it for his own.
Not in a jerk way, of course. Jon Snow would never take what wasn't his — but he did become King In The North after making his way back to what many consider his rightful home. This is what makes everything in Season 8 so complicated. As King In The North, Jon's first move was to tell Sansa that he was headed to Dragonstone to bend the knee to Daenerys in the name of asking for help to fight the Night King and White Walkers. (Dragonstone is full of dragonglass, and plus, Dany had made her name known as a major boss.)
But things are complicated now. Jon went out to Dragonstone to prove to Dany that they needed to combine forces to fight the Night King, and she was quickly sold on the mission. She was also sold on Jon Snow's emo ways and his dreamy locks — the Season 7 finale showed them having sex on the way back to Winterfell to take on the Night King.
That would be all well and good if fans didn't also learn in the Season 7 that Jon and Dany are related. Fans knew going into the Season 8 premiere that Jon is the son of Lyanna Stark and Rhaegar Targaryen (and Rhaegar is the older brother of Viserys and Daenarys), which means that Dany is actually Jon's aunt.
Not a good look. So when Samwell tells Jon that he's been making sweet love and war plans with his biological aunt, it makes total sense that the poor guys was perturbed and claims that he's no longer the king of anything.
Jon's title and his ancestry are important when it comes to this final season. There are a ton of theories out there that because Jon is both Stark and Targaryen he can take on a ton of roles. He rode a dragon in the Season 8 premiere, which is promising when it comes to thinking about how the big battle will go down this season. It also means — if you believe the theory that the Night King is a Stark — that he might take one for the team somehow and end up a supreme White Walker.
Anything can happen. But for now, Jon is helping the rest of his allies in Westeros fend off the dreadful winter as King In The North, no matter what he says when he's cranky.