Takes place after 7x23. Spoilers up to that episode.
Disclaimer: None of the characters are mine.
The portal was open. Blissfully, blindingly open. Sam had done it. Dean had no idea how, but there it was. He could practically smell the earth just on the other side, could almost taste the sweet, fresh air. Something told him this was no trick, no illusion. He turned to Cas with a smile- the first time he'd allowed himself one since being trapped in Purgatory. Castiel was gazing back at him with a corner of his lips upturned. The angel nodded, as if telling Dean it was safe to proceed. Dean took a few steps forward and was about to reach out for the very edge of the portal when he glanced back. Castiel was still standing exactly where he was before, that minute smile still plastered on his pale face and his eyes still firmly set upon Dean. Dean's grin faltered.
"Cas! Come on, this is our way out!" he yelled back, beckoning the angel to follow him.
Castiel shook his head. "Go, Dean. Go find Sam," he said calmly, and Dean strained to hear him over the sound of rushing wind that the portal produced.
Dean ran back to Castiel, grabbed his arm, and pulled roughly. "This isn't funny, Cas. Let's go." Again the angel shook his head. "Damn it, come on!"
"The portal could close at any moment. Please go while you're still able," Castiel continued in the same calm voice. The edges of the portal were already shrinking, but Dean refused to look anywhere but at Castiel. He wasn't leaving without him. He pulled on his arm again, yet the angel remained as still as stone.
"Why won't you come with me?" he asked in confusion. They had been planning and plotting to find a way out for what seemed like months. They had no idea how long it really was, for time was a nonexistent entity in Purgatory. And now that a way out was there, literally right in front of them, Castiel wouldn't move.
"I belong here, Dean. Here I can pay for my mistakes. I should never have walked out of that lake, and so this is where I must stay as an alternative. I have caused enough harm to both you and Sam," he replied, looking down at Dean's hand where it was gripped tightly on his left arm.
Dean's grip tightened to what would have been painful to a human and glared angrily at Castiel. "You pulled me from Hell. You rebelled against Heaven. You saved me and Sam more times than I can count. You've died for me, and not just once. You're the only angel I've ever met that hasn't been a complete and total dick. You're a part of my family now, and a couple of mistakes, no matter how colossally stupid, can't change that. Not really. So yeah, I'm still a little bitter about it. But I am not going to let you die in here. I'm tired of losing my family, Cas. I am not going to let you leave all over again. Now get your feathery ass through that portal or so help me I will drag you out."
Castiel stared at Dean for a moment longer and Dean saw the conflicting emotions in his eyes. Finally, with one more tug, Castiel moved with Dean. Quickly man and angel exited through the portal and were thrown onto the soft earth on the other side. The gaping portal quickly shrunk into nothingness and Dean coughed, rubbing the back of his shoulder where it had impacted the ground. He turned to see Castiel lying next to him, eyes closed. Dean panicked for a moment, placing his hand on the angel's chest before Castiel's eyes opened calmly to look into Dean's.
"Thank you," he said gruffly, placing his hand atop Dean's. They smiled and soon heard the distinct shouting of one Sam Winchester as he came rushing toward them. They were finally home.