Dean has basically had a continual goal of NOPE NOPE NOPE anything but this for Cas
Even if it means the pain of taking himself (as he is poison) away from Cas for his own good
But Cas won’t leave him even at his worst
So of course he feels all the pain and anger when Cas does eventually nearly then actually die and it’s because he was poison, the things he did with him changed Cas, Cas stayed with him through all this and he led Cas here
And then here
And when he does eventually realise and admit that it wasn’t Jack’s fault, that he didn’t brainwash Cas, that Cas chose to do what he did when he forgives Jack and accepts him as family it only serves to make it worse. If Cas’ death is not Jack’s fault then whose is it? If you’re thinking Dean’s self worth / I’m poison / everything I touch turns to ash / I’ve done nothing but cause Cas to fall, lose his purpose, his family, his entire existence, he was so depressed and ended up caring so little about his own life that he let Lucifer in then all this / specifically thinks he’s the one who led Cas here to his death from only a few episodes before this is somehow important in season 13 again… I think you’re damn right.
So no wonder he continues to try to hold Cas at arms length for Cas’ own good when he gets him back in season 13. Even though all Cas wants is to be allowed to stay with him and be closer. Dean thinks he is doing this for Cas because he will only make things worse for him, as proved only recently.
He still thinks Cas would be far better off without him.
Meanwhile Cas thinks Dean pushing him away and keeping him at arms length is just because he doesn’t want him not because Dean can’t bear to lose him at all and will give up his own happiness to maintain what he thinks are Cas’ best interests.
I’ll take miscommunication and self loathing tropes for $200 Susan.