Pointedly not thinking about how Dean's bond with Castiel is literally one of the first things in the show that is inherently Dean's own.
Dean is a man without a real home, who never got to build a life of his own, who adopted his father's quest and rage and whole persona before ever really being himself, who for most of his life was a mirror of everything John became after Mary's death and who always perceived his own self as lesser than what his father was.
Beside the obvious it's in the smaller things, too. His taste in music? John's. His clothes? John's. His car? John's. Even the Samulet was meant as a gift for John initially. Dean spent the majority of his life trying to be his father, burying his own self beneath the weight that was his father's burden, something John made him carry in the first place. Via Chuck's plan Dean was on borrowed time even before he was born, because when Mary made a deal that would eventually taint Sam she signed away Dean's life, too.
On his father's orders Dean lived his life to protect his brother. He sells his soul for him. He later learns he was destined to do it, too, meaning not even his soul was really his own to begin with, since he was always meant to trade it for Sam.
All his life, Dean didn’t have something of his own, a choice, a life, a sense of who he is or who to be. Then, he went to hell and that was it. Or so he thought.
An angel, literally one of the most powerful beings in creation and potentially the only one who has an even bigger chip on his shoulder than Dean, who also has never had anything but his own sense of duty, his orders, the bigger picture.
Dean and him have a connection. From the get go it's different than anything Dean's ever known. Suddenly, there is someone willing to do everything for him, to make the heart choice for him instead of the hard choice for the entirety of the world.
Castiel chooses Dean when his whole life Dean watched people choose anything but him, when his whole life Dean chose anything but himself.
Dean lives his life thinking he is nothing but the sum of his parts: a brother, a hunter, a killer, a copy of his own father and then along comes this angel, who not only rescues him from hell and tells him he is righteous and good, but who eventually also chooses him over heaven, who chooses him again and again and again in the same way Dean always chose Sam and will always choose Sam, but Castiel doesn’t care about that. He cares about Dean unconditionally and demands nothing in return.
And this is not something Dean has to share with anyone, and not something he has to give up for Sam either. For the first time in Dean’s life something is about him and only him. Someone cares about him first and everything else second. It’s novel, it’s unheard of, and it's not tied to some weird sense of duty or a condition either. It's not in Castiel to care about Dean in the way it is in Dean to care about Sam, but Castiel does it anyway.
And Dean does the only thing he knows how to do: He fights the connection, seeks out the metaphorical a bump in the road, looks for conflict wherever he can, thinks: “I knew he would leave.” when Castiel walks out on him for one reason or another, because that’s how it always is. People always leave.
Dean doesn’t get to have things. He doesn’t get to have meaningful bonds and he certainly doesn’t get someone willing to sacrifice everything for him. People don’t sacrifice themselves for Dean. It’s always the other way around. He always loses things because they are never really his in the first place.
But instead of leaving for good Castiel keeps coming back, even when Dean tries his hardest to push him away, even when the whole world tries to keep the angel from returning, even when they both make mistakes that should damage their bond beyond repair. Every time Dean thinks “I lost him.” Castiel finds his way back to say:
“You still have me. You’ll always have me.“
