IF (a very big if) the satan baby isn’t controlling/manipulating/brainwashing/possessing Cas in any way and Cas is doing this out of his own free will, this is the way I’m choosing to see it:
Angels are beings of faith. It’s in their nature to always want to have SOMETHING to place their faith on. Cas, who’s had that faith in God, placed his faith on Dean and Sam. From s5 onward, he placed it on Dean.
But now Dean isn’t someone he worships that way anymore, which is a really really good sign that Cas finally sees both of them on equal footing instead of putting Dean on a high ass pedestal as someone who’s impossible to reach. He worships him in a different way now, so he needs that satan baby to place his faith on.