Jon Snow has plenty of reasons to be upset with Theon Greyjoy, but the King in the North offered him some solid advice during the Game Of Thrones Season 7 finale. By telling Theon that he doesn't have to choose between being a Greyjoy and a Stark, Jon proves that he's still a Stark on Game Of Thrones even after he finds out that he's a Targaryen. While it was just another moment of screaming at the screen due to the fact that Jon Snow still doesn't know who his real parents are, it was also an important foreshadowing for when he does find out. (Unfortunately, Sam and Bran weren't able to tell him before Jon had sex with his aunt.)
While Theon has a lot to apologize for to Jon, like betraying the Starks by taking over Winterfell and pretending to kill Bran and Rickon, Theon has changed a lot after being tortured by Ramsay. Still, Theon succumbed to his cowardice once again when he abandoned his sister Yara after Uncle Euron attacked them. But, seeing Jon continuously do the right thing inspired Theon during the Season 7 finale and he had a heart-to-heart with Jon about how he has spent his life being conflicted about whether or not he should be a moral Stark or a harsh Greyjoy.
Jon's wisdom to Theon that he'll always have Ned Stark inside of him and that he is both a Greyjoy and a Stark proved immensely helpful as it encouraged Theon to go save his sister Yara. But far more meaningful to the overall story of Game Of Thrones is that Jon will be needing to give himself pretty much the same exact pep talk once he finds out he is not Ned Stark's son. Bran and Sam spent the Season 7 finale decoding Jon's past to learn that he is the legitimate son of Lyanna Stark and Rhaegar Targaryen — meaning he is both a Stark and a Targaryen, along with being the rightful heir to the Iron Throne. Jon has always grappled with his Stark identity since he thinks he is a bastard, but knowing he's actually a Targaryen is really going to mess with his mind — and that was true even before he unknowingly slept with Aunt Daenerys.
Although Dany is good, the Taragaryens were never known for their kindness like Ned Stark was. Jon prides himself on doing the right thing, like his assumed father always did. He's absolutely going to have a hard time accepting that the man whom he thought had kidnapped his aunt was really his father this whole time. And while he'll surely take some comfort in finding out that his parents were married and truly loved one another, knowing their love caused a war won't help. Even worse is that this upstanding gentleman just accidentally had sex with his relative, making Theon's whole existential crisis super insignificant in the scheme of things.
So yes, while this moment is a turning point for Theon, it will prove even more important for Jon once he meets up with Bran and Sam at Winterfell. And though he will be dealing with an absurd amount of emotions about his parentage, here's hoping he can look back on his words to Theon with some clarity and know that Ned Stark will always live on in him. Because, just as he tells Theon, Jon Snow/Aegon Targaryen can balance being both a Targaryen and a Stark while remaining true to himself.