So I’m pretty sure the person Dean is talking to here is Cas. It can’t be Sam as even with his new short haircut that much of his ear and neck wouldn’t be visible from this angle, and I also spy a tiny bit of a tan collar there in the corner. And you know what this reminds me of? That scene in Road Trip where he’s talking with Cas and says “so what you’re saying is we’re both a couple of dumbasses?” and even with everything going on he’s still so soft and calm around him.
I’m gonna stick with my theory that Dean will not be cured all the way in 10x03, but Sam will administer human blood to him that will act as a temporary fix until they can figure out what to do about the MoC. I don’t know if Cas will be present for the curing, but it looks like he’s in the bunker at some point as I believe they’re standing in Dean’s room in this scene. IDK I’m just emotional about the way he talks to Cas and the way he is with Cas and the two of them standing in his room talking goodbye friends I am gone.