The Cas-Jack relationship is a Dean-Sam codependency mirror.
Let’s say that Jack = Sam Cas = Dean and Jack’s powers, which compel Cas to act as Jack’s parent = John and John’s parenting, which compels Dean to act as Sam’s parent
Last season, I was cautiously wondering if the show was making that parallel, but then 13x01 aired and the word “brainwashing” was spoken and I was sold. That’s a word that’s been connected with John: in 10x03, Demon!Dean says, “Dad? Oh, there’s a prize. There’s a man who brainwashed us into wasting our lives fighting his losing battle!” and then, two episodes later, John’s parenting is described like this:
He trained us both to track and hunt and kill
He took away our own free will
That’s in reference to the hunting life. But Dean’s compulsion to put Sam’s life ahead of his own life has been described like that, too. In 3x10, Dream!Dean says, “You've got nothing outside of Sam. You are nothing. You're as mindless and obedient as an attack dog … Do you even have an original thought? No. No, all there is is, ‘Watch out for Sammy. Look out for your little brother, boy!’ You can still hear your Dad's voice in your head, can't you?”
So when Cas is described as being “brainwashed” (13x01) and “sock-puppeted” (12x20), I have to pay attention. Especially when we see what he’s being “brainwashed” into doing: becoming the parent of a child that isn’t his. Putting that child’s life above his own. That’s exactly what happened to Dean. He had to become Sam’s parent because John couldn’t do it: “Dad was just a shell… And I—I had to be more than just a brother. I had to be a father and I had to be a mother, to keep him safe. And that wasn't fair. And I couldn't do it” (12x22). To keep Sam safe, Dean sacrifices his childhood, and he sacrifices his “second chance” at a normal life at Sonny’s (9x07), and he sacrifices a lot of good relationships (Robin, Sonny and Benny to name a few), and he sacrifices his soul (2x22), and he’s willing to sacrifice his life if that’s what it takes (11x17).
Castiel: Kelly… I promise you—I will do everything. I will give my life for your son. And I will raise him. And I will make him someone you will be proud of (12x23).
Cas, having been mindwhammied by Jack’s powers, is suddenly willing to protect Jack at the expense of his own life; at the expense of his most cherished relationships (abruptly going from “Okay, we’ll talk” to ~gonna knock you out against your will, leave you abandoned in a park at night, and go radio silent~ ); and at the potential expense of the entire world. It’s nothing that Dean hasn’t already done for Sam. It’s presented as just as horrifying, frightening, and wrong as Sam and Dean’s codependent extremes have always been.
Cas views his compulsion as “faith”:
Dean: Are you okay? Castiel: I am. I've been so lost. I'm not lost anymore. And I know now that this child must be born with all of his power. Sam: You can't actually mean that. Castiel: Yes. I do. I have faith (12x19).
Which is exactly how a younger Dean relates to John:
Sam: I don’t understand the blind faith you have in the man. I mean, it’s like you don’t even question him. Dean: Yeah, it’s called being a good son! (1x11)
No wonder he’s so anxious about Cas’ “faith” in Jack’s powers.
Cas: I have faith. Dean: Really? In your unborn baby-God? Cas: Yes. Dean: Well, then, you're a dumbass. (12x23)
Meanwhile, young Sam and Jack are largely innocent and ignorant of all of this. Young Sam obviously has no say in how he’s raised. That’s John’s decision. Jack says that he “chose” Cas as his father, but as of 13x01 it looks like he can’t consciously control his powers and that he views them as separate from himself, so, as far as we know, he never consciously forced Cas (or Kelly) into doing anything. He just wanted Cas as his father, as simple as that, and his powers took care of making sure it happened, all on their own. None of this is Sam or Jack’s fault—they’ll both just need to become (more) aware of the problem and to do their part to help fix it.
Jack and Cas can still have a relationship, just like Sam and Dean can still have a relationship… But they all need to deal with the unhealthy foundations of those relationships and make them into something better.
What’s happening to Cas right now is a great metaphor for what’s happened to Dean—and Sam—for their whole lives. Imo, it bodes really well for a good resolution to their relationship with John and to their codependent relationship with each other.