There'll be a fic later tonight once I get off my work computer and back home, but I couldn't pass up an excuse to finally write a bit of meta on why I ship these two so damn hard.
Spoilers for season five in both text and images.
Forgoing Memento-style shenanigans, I prefer to tell a story in chronological order wherever possible. Because while it's easy to jump to where we are now, with awesome fanfic, glorious meta, beautiful picspams and canon fanservice up the wazoo, whenever I trace my love for this pairing, it always points to the same moment;
"You don't think you deserve to be saved."
I'm a shipper of most everything and in Supernatural fandom I'd been an enormous fan of Dean/Sam fanfic up until that point (despite wholly and utterly sucking at writing it myself), so the fact I jumped with joy at another pretty thing being added to the cast wasn't so odd. What made Dean/Castiel stand out was the intensity with which it hit me; normally with ships I'd start from "okay, these two would be fun together, so how would the canon for them work with that idea?" and think things through logically, even if the end result happened to be "cracky pairing is cracky. Write crackfic".
With Dean and Castiel, in amidst the overwhelming squee of "THEY HAVE ANGELS NOW. ANGELS WHO EPITOMISE HOW I ALWAYS THOUGHT ANGELS SHOULD BE", that one line was a punch to the gut - because in one line someone had finally, finally seen to the core of Dean, seen the damaged, self-hating creature he was, and said it to his face with this detached curiosity and the implication in it that he was wrong to think so badly of himself, and I was just sold in an instant.
One and a half seasons in and I am utterly convinced that Castiel still sees something good in Dean under the self-sacrificing self-hate, and that he's gone from detached curiosity about it to doing anything to protect it, up to and including turning his back on heaven and dying. Hell, Castiel even gives us the short summary himself -
I killed two angels this week. Those are my brothers. I'm hunted, I rebelled, and I did it, all of it, for you.
Even broken and lost future Castiel still obeys future Dean's orders as if there's some part of him - a part of him his conscious self clearly regards with disdain and self-hate of his own - that perhaps future Dean still has the potential for better.
Castiel's slide towards humanity is a fun plot point of its own but to me everything still hinges on Castiel having faith in two things - God, and Dean - to the point where he'll defy Heaven itself out of his love for both of them.
And to me, there's no doubt that it is love. I honestly believe Castiel loved Dean from the start, and that what we've seen is a transition from the generic idea of love, the love thy neighbour as yourself detached and vague affectionate sort of love, to something filial at the least - and in my dreams and in fanworks, romantic.
Dean is where things get really interesting though, because Dean is this tragic web of complexes and obsessions and yet almost from the off, Castiel has thrown him and his preconceived ideas about people for a loop. Watching Dean with Castiel almost hurts because even without seeing the other seasons the effects of his codependence is clear; but running all the way back to season one and the episode Skin we know that Dean, in adult life, has outright rejected the idea of having friends, though not without some resentment.
"You got friends, you could have a life."
Might be a shapeshifter saying it at the time, but it certainly echoes Dean's words throughout the episode and, indeed, throughout the series.
Across the seasons, it's not hard to see why; although Sam opted against telling his Stanford friends about hunting, whenever Sam stops to explain hunting, people almost always believe him. Dean gets rejected and yelled at nearly every single time - he even jokes more than once that Sam can just use his puppy-dog eyes and people believe him. Dean might be able to talk casually with people easily, but Sam's the only one to strike up friendships with relative ease.
Watching Dean with Castiel is almost like watching therapy; we see him treating Castiel like a little brother when tidying him up, a big brother when asking for help, a counsellor when confessing how, ultimately, he doesn't think he can stop the Apocalypse or save the world. Ultimately, Castiel is Dean's first real friend; the hunters his surrogate family. And a crash course in friendship isn't easy; Dean sometimes tries too hard and gets overly dependent at points, while at others he's neglectful and ignorant. But there are moments where they click - brief, wonderful moments like the prayer scene.
Click to view
In canon, I believe Dean is starting to mean it when he calls Castiel a friend - it's just that he's so unsure of the boundaries of friendship that he still does things quite regularly that seem ungrateful or ignorant. He's actually lucky that Castiel won't take much crap from him, as it means instead of losing his only friend when he screws up, Castiel's there to outright tell him when he's being thoughtless.
I don't think Dean, in canon, is in love with Castiel in any traditional sense; but I truly believe he could be. I think Dean is still overwhelmed by Castiel - by the idea of him, who he is and what he represents - to the point where he's in denial. You can scarce blame him for the struggle; even leaving aside the obvious trust issues that Dean is more than entitled to, he's spent decades suffering in Hell only to return and be told there's a Heaven he could have gone to if he'd just let his little brother die.
The End is a mixed bag episode for what it shows and implies, but I think after it, Dean's finally started to see that Castiel's angelic origins aren't just something to rail against; he's seen how very human, how badly human Castiel can become. Now there's finally room for him to appreciate Castiel as a friend and an angel, and with Dean starting to accept Castiel as a whole, that opens up a possibility for love where we're not just taking canon and playing with it, but we're taking canon and writing for something that's already there. And we certainly know Dean already asks one thing of Castiel:
"Don't ever change."
It doesn't come much more epic than one half rising from Hell and the other falling from Heaven, be the end results friendship or something more, and seeing anyone help mend Dean after what he went through in Hell? Is pretty darned uplifting, all things considered, whether you like the pairing or not.
(And for the record, Sam's a forward-thinking former college student; I think he could approve. Eventually)