Reason # 6: RIVALS, PRIMAL REACTIONS and CONTEXT
About Jon’s explosive reaction people have argued for and against it being out of jealousy over Sansa. I personally think Jon’s entire reaction was half conscious and half sub-consciuos. Again, the deep love Jon and Sansa have for one another is consciously framed by their awareness of being half-siblings - it’s their “explanation” (justification) for why they feel so close and attached. Even if they perceive the awkward sexual tension on some level, they would never consciously pursue even the thought of having “romantic” feelings for one another. Until they know that they do not share the same father, which will be such a huge game changer, I can’t wait for it!
However, Jon’s reaction is still over the top, even for a loving brother. It’s a very, well, primal “male” reaction, if you get my meaning. And by that I also mean that it was an unconscious and completely *uncontrolled* reaction that came from somewhere deep down within, some place that is yet unexplored and hidden. Context is so relevant in Game of Thrones, just to keep track of everyone’s motivations and what drives them. We see everything between Jon and Sansa now through the lens of their being half-siblings (or think so anyways). If this were not a factor at all, if Jon was like Theon, a warden of House Stark or something, all their interaction we saw so far, every romance trope, every lingering look, everything would be *clearly* interpreted as romantic and a majority of viewers would concede that they will likely end up together...
The revelation of Jon’s parentage will change the entire context of the Jon/Sansa relationship and dynamic into something like this. They will still be cousins, but it will change the way they perceive each other. And the one person, besides Bran, who I think knows the real “context” in the show already is Littlefinger. If he thought Jon was *only* a concerned brother, we would not see the cogs in his mind working like they do when he steps out of the crypts.
If you desire (I hesitate to write love because it’s Littlefinger and he never loved anyone but Cat) a woman like Littlefinger desires Sansa - to the point that his obssession with her mother has shifted to her - then you are consciously aware of your (sexual) rivals. Jon’s reaction is only that of a jealous lover seen within the *right* context - and Littlefinger has the context to identify it as such.
He looks at Jon as if he suddenly sees him clearly for the first time. He knew who he truly is before --- but (as other posters have pointed out before) now Littlefinger knows what Jon truly wants, the only thing he truly wants - he knows it even *before* Jon will know it on a conscious level himself.
And that’s why he now sees Jon as a rival, because he knows that Jonsa is a *real* possibility.
It is in fact exactly like that LF quote from the books others have brilliantly pointed out: “And when you know what a man wants [Jon: nobody touch Sansa, Sansa safe, Sansa happy] you know who he is [Jon: the Targaryen prince] and how to move him [Jon: literally anything to do with Sansa]”
