Okay, how about this?
Dean and Cas had an epic love affair in Purgatory. It was tender and touching and beautiful and everything we want. Right up until the end Dean was always ‘I’M NEVER GOING TO LEAVE YOU CAS! I’LL NEVER LET GO!’
Then they are opening the portal and Dean is letting Benny hitch his ride, because Dean promised. Only just then some leviathan jump out of nowhere thanks to a planned plot by Benny.
A Dean with Benny riding shotgun grabs for Cas, desperate.
BUT, Benny not only takes over enough to ‘incept’ Dean into letting Cas go, Benny also Eternal Sunshines all of Dean’s memories of his and Cas’ beautiful love story, so what Cas sees at the end is a Dean who DOESN’T EVEN REMEMBER THEM LOVING EACH OTHER, which is why he looks so shocked and hurt and scared.
So now Dean is walking around thinking there’s something MISSING… but he can’t for the life of him remember what. And thinking about Cas too long makes his head all fuzzy and confused and it HURTS, so he’s decided not to try.
And when Cas comes back he’s gonna go inside Dean’s mind and tentatively open up all the memories of their relationship, one touch and kiss at a time, until Dean remembers it all and breaks down at Cas in apology for having forgotten all this time <3