"SPOILERS - This is set after season 12, episode 3 and before episode 4. I just felt so sad for Dean, I wanted to do something for him.

"Cas materialised next to the bunker door. He was still surprised that Crowley had sent him on his way without arguing. He dusted his coat, looking this and that way. Even more surprising: the demon hadn't followed him. Castiel opened the door and walked in. The sound of the locks echoed in the darkness of the bunker. He frowned. The lights should have been on. He could feel both Dean and Sam's presence. He couldn't feel Mary, but he also couldn't sense any one else with an evil intent. Warily, he walked down the stair and into the foyer. He could sense Sam was nearby, but for some reason, he felt he shouldn't call his name. He made his way past the first alcove, then the second and stopped at the third one. His back against the shelves, Sam sat on the floor hugging his legs, his forehead resting upon his knees. His shoulders shuddered from time to time. Castiel stared at him, unsure of what to do. His relationship with Sam had always been awkward, despite what Dean thought, and he wasn't sure if (or how!) he should comfort the man. As if on cue, Sam lifted his head. His eyes were puffy. He gave the angel a sad smile before wiping his face with his sleeve. He made to get up. Castiel stopped him with a gesture of the hand.

"Did something happen to Mary?"
Sam shook his head. He started to talk but the words died before they came out. He squeezed his eyes and pinched the bridge of his nose, as if to stop the tears. It didn't work. He cleared his throat."She left. Said she needed some time on her own. To adjust."
Castiel felt a heavy weight where the stomach of his vessel was. It was uncomfortable, unpleasant. He didn't need his angel senses to understand that fear was slowly taking hold of him.
"Where is Dean?"
"I'm not sure. He said he was going to his room."
"Castiel turned around but Sam got up and grabbed him by the arm. Castiel had always been amazed by Sam's ability to recover and he wasn't disappointed now. Yet, the man's eyes still betrayed his anxiety.
"He'll think it was his fault. You know him, Cas. I would've talked to me, but you know how he... we are."
Castiel nodded.
"Will you...?" the angel's words trailed.
Sam tucked his hair behind his ears and wiped his face anew.
"I'll be fine." He paced. "I'm fine."
He wasn't but he would be. Dean was another story. He needed to find him before he used this new nail to crucify himself for this new development. He scanned the bunker. Dean's presence was growing faint. He knew exactly where to find him.

The door to the storage room had been left slightly ajar. It opened without a creak. Dean's presence lingered in the air. The back wall was covered with rows upon rows of shelves but Castiel knew this room too well. His fingers soon found the latch that triggered the hidden door's mechanism. The wheels and cogs clanked as the shelves moved aside revealing the dark room. Castiel didn't need his eyes to adjust to the darkness to see Dean sitting in a corner in a similar position he had found Sam earlier. Something crunched under the angel's feet as he advanced. He suddenly realised that the floor was littered with pieces of glass, shards of wood, and other broken object. The smell of blood hit him. He rushed to Dean.
"Dean!"
Just before he could reach him, his friend lifted his face and glared. His eyes were dry. Castiel stopped in his track. He swallowed hard.
"Go away, Castiel."
"Dean. You're hurt. Let me heal you, at least."
The man clenched his jaw.
"Leave."
"Dean..."
"Dean took a deep breath.
"Cas. I said leave.
"Listen, I don't know what happened but you can't blame your..."
The words had barely left Castiel's lips when Dean's fist connected with his face. He reeled backward and didn't have time to steady himself before the man shoved him with all his strength. Castiel hit the wall so hard his vessel would indubitably have been knocked out. Dean stood in the middle of the room, panting.
"Now leave." His eyes avoided the angel's.
"No."
Dean glared at him. He roared and pounced on Castiel. The angel braced himself. Dean came at him again and again. Again and again, Castiel stood back up and took what was coming. He didn't avoid the punches. He didn't move out of the way. Dean's screams weaken with his strength, his voice gradually turning hoarse, cracking. A good twenty minutes had passed when Dean finally fell down to his knees. He hunched forward, his bruised fists cradled on his lap. His breath was laboured, halted. Castiel knelt next to Dean and rested his palm on his back. The man tried to shrug him off put the angel put his hand down again. This time, Dean didn't resist. Castiel could feel his friend's body tense with each sob he tried to suppress. Gently, Castiel grabbed Dean's shoulders and helped him sit upright. The angel's heart sank. Dean's face glistened with tears and his mouth was twisted in a painful grimace. Their eyes met for a moment but Dean immediate averted his gaze. Before he knew what he was doing, Castiel was wiping the tears from Dean's face which drew forth more tears. Dean tried to turn away but Castiel held him by the shoulders.
"I know what you think. Dean... Look at me"
"Dean lifted his face. He was struggling to keep the storm at bay.
"It's not your fault."
Dean shook his head. Castiel cupped Dean's face in his hands.
"You can't blame yourself for everything, Dean."
"Why?" Nothing more than a whisper. "Why do I push them away?" His voice shook. "What is wrong with me, Cas? Why is it that they always end up leaving?"
With those words, the dam broke. Dean covered his face with his bloody hands and leaned forward, sob wracking his body. Castiel had seen Dean cry on few occasion, and he has sensed him do so on a few more, but never had he sensed that much despair in his friend. He pulled Dean to his chest and held him tight. Dean grabbed onto the front of the angel's coat and wailed, giving in to the embrace completely. Castiel rested his cheek on the top of Dean's head. He didn't say a word. He didn't make a sound. The ball in the pit of his stomach grew heavier, as if Dean's body was emptying itself of its sadness and in turn, filling his. Angels didn't cry. Angels didn't love. Then why were Castiel's eyes brimming with tears. Why did he fear that this moment would destroy everything?