Archangels are absolute

Prequal

Welcome to my version of Cas raising Dean from literal hell. I promise this is relevant to the story.

Cas fights his way with his army through hell, finding Dean waiting, covered in blood and god knows what, to resume torturing once his handler arrives. 

He refuses to believe Castiel at first, thinking it's his time to get tortured a little, cursing Alastier for going back on his word and cursing himself for thinking he would. Cas takes a look at Dean's soul, it's been marked and scarred from so long. 

Cas heals his soul, putting the pieces back into place with pieces of his own grace. It's temporary at least, it'll be more permanent when he can get Dean back into his body. Then Cas drags an unconscious Dean away from hell. 

But things get complicated, there are too many demons for Castiel to get Dean and his kin out safely.

Cas orders everyone to return to heaven immediately, he knows a special route that'll take him and Dean back to earth but it'll take a while.

They only make it to the figurative dead man's zone, where desperate humans try to escape hell only to be brought back, the home of crossroad demons. 

But they have to stop from time to time for Cas to continue healing his soul, keeping demons and ghouls that are trying to take back Dean at bay, and apparently part of Castiel's job is to play therapist and convince the freaking Righteous Man he's worth saving. 

Cas may snap and we'll um.. exclaim about how much Dean has sacrificed for people and show selfless he is and how he took care of Sam and didn't scream at his dad because there were more important things and just wanted his family to be together and happy even if it means he has to work at the garage during the day and cook in a diner in the afternoon and almost never go to school but still helps Sam with homework and teaches himself from books and could have done so much in the human world and had a normal 'apple pie' life if he had just been a bit more selfish but he chose to help people instead and he deserves to be saved damnit.

Dean is coming to terms with everything, and taking (angrily ranting) to an archangel that hasn't even seen Star Wars but somehow knows almost everything about him is strangely helpful. 

Turns out Cas could use some therapy too, heaven isn't all it's cut out to be, especially for the angels. 

As they travel Dean catches Cas watching the human souls get tortured, his eyes lingering on them and this look in his eyes when he hears an especially loud scream. Dean listens when Cas explains his concerns about what exactly some of these people did to deserve it, and whether or not they should be given another chance. But he can't do anything without getting into huge unimaginable trouble and the souls would only be sent back to hell. 

Dean asks about watching humanity grow up, where he was when what happened. 
Cas tells him about growing up him heaven, how his brother (Gabriel) raised him much like Dean raised Sam. 

They talk about the dark ages, they were caused by a demon plague in heaven but once things were okay the angels were convicted that humanity was hopeless. Castiel healed the black plague and tried to inspire people to be better. Eventually, Gabriel sought Cas out and they worked together to bring the Renaissance.
Of course, many other angels jumped in once it kicked off to take credits.
Dean gets pissed when he finds out God allowed this and hadn't been around in a long time.

Cas tells him about watching humanity grow up along with him. Being confused between being a protector/shepherd and the cold-blooded soldier heaven wanted. He admits God had been absent for so long that Castile hasn't even met him before

Dean gets kinda pissed when he finds out that asshole trickster is Cas's brother and an angel, especially with the whole Groundhog Tuesday debacle. 

They're still them, Cas is still awkward and doesn't get pop culture references. Which is infuriating to Dean since he has to figure out how to talk in his own words. Dean can't express his emotions, tries to act tough, is self depressing, internalizes everything (*cough* internalized homophobia *cough*), and is so so tired. 

They finally get to an exit, Cas explains that to put Dean's soul back into his body it'll be very painful and confirms they'll probably not see each other for a while. 

The demons come, attacking in a last desperate attempt to keep Dean in Hell. Destiel fights back to back, almost like they've known each other for years (well, time in heaven and hell is pretty weird). 

They get separated for only a few seconds and Dean gets hit in the head making him dizzy and unfocused but he's still conscious.

Noticing this along with the fact they're surrounded, Cas wraps Dean in his arms, kisses his cheek, and raises them from hell in a surge of power.

The amount of power he has to use drains him, marks Dean, and causes Dean a lot of pain. 

Castiel realizes something, it might have taken (months, year/two it's up to you) to get out of hell for them, but to a human, it would be less than a week
(did the math, one day on earth is about 121 days in hell (40 years times 365 days a year = 14600 days divided by 139 days between May 3rd aka the day Dean died to September 18th aka the day he came back = 105.04 hell days for every earth day))
That amount of experience and mental progress and feelings would tear him to pieces.

So he wipes Dean's memory and puts dean's soul back into his newly reconstructed body.

The force of it blows back every tree in a mile radius. Despite what he just had to do, Cas yells with joy when Dean starts breathing again. Every angel and Nephilim in the world, heaven, and even hell.
Cas watches Dean until he gets to Bobby's, being extra careful after the gas station.
He couldn't help it, seeing the mark on Dean's body. He couldn't help himself, he had to take a closer look. Make sure no mistakes were made (sure Cas).

So Dean went back to earth and Sam while Cas went back to heaven alone.