Sam didn't know how he could have missed the signs. That's really the only thoughts that race through his mind as he drives back to the bunker with his brother's broken, empty body in the Impala's back seat. How had he not noticed that his brother was quickly plunging off of a cliff that he would not be able to drag him back up from. Had his act really been that convincing or had he really been so angry at his brother to notice that Dean was falling apart. Dean had known almost instantly when Sam had been hiding the effects of the trials and yet he had failed his brother when Dean needed him the most. Worst of all, Sam knew that his rejection of Dean's actions after Dean had literally done everything he could think of to save him when he was dying was what had set Dean off that cliff. What had caused him to plummet so low that by the time Sam realized what was happening, Dean was dying in his arms.
Dean had always been Sam's rock. His fall back place whenever things got too hard or too deep. Yet when his brother had need the same from him, Sam had pushed him away. Had said to his face that he wouldn't have gone to the same lengths for Dean. Sam might as well have placed that mark on Dean's arm and not Cain, because the one who had cause the chain reaction that led to an empty corpse in the back seat was him. His own blind righteousness. His own anger. His own stupidity. Sam was the only one to blame for the fact that he was carrying a corpse to the only place that had become a safe place for them.
Sam kept going over and over Dean's actions and looking for clue-ins and they were deeply hidden but they were there. The strained smiles that never reached the too distant eyes. The shoulders that had always been so strong, beginning to slope in towards his chest. His movements being just a fraction of a second slower than usual as if Dean wasn't sure if he wanted to stop the creatures from ripping him apart. They were all so obvious now to Sam and he couldn't understand what had blinded them to him before. Then his eyes landed on the army ranger that was still stuck in that one space in the rear driver side door and Sam knew. Knew what had kept him from seeing the deteriorating state of his brother. It wasn't the anger at his choice to keep Sam alive or even the distance that Sam had been striving to put between them. No, it was the admiration and belief that nothing could ever take down Dean Winchester. His blind faith in his brother's abilities and the hero like reverence that Sam still clung to from childhood for Dean, had kept him from seeing his brother destroy himself.
Though Sam supposed it made sense in a twisted and dark way. The only thing with the power to destroy Dean was Dean himself. Nothing else had the power to destroy him except, Sam amended thinking of how things he had said to Dean had literally shattered the man on several occasions, Sam. So Sam and himself had been Dean's downfall but Sam was determined that he would also be one of the reasons that Dean would return. If Sam couldn't chose to die on his own terms without his brother dragging him back, then neither could Dean. Besides, the world needed more men like Dean, the man was the best person Sam knew and that wasn't an exaggeration.
We've seen sorrow and tragedies too awful for words. Fought creatures that are the incarnation of evil. Lost our mother. Lost our Father. Lost Bobby. Lost Ellen, Jo, and Ash. Lost Kevin. I lost Jess and you lost Lisa and Ben. We have both almost lost or lost each other one way or another. We cannot survive alone, Sam decided. We have to stick together and he would bring his brother back. Once he had done that he would work on fixing the damage he had done to the man he considered his best friend but until then, he would focus on ignoring the fact that his brother's cooling corpse was in the back seat. Instead, he would imagine that he would see Dean, alive and whole, just as soon as he arrived at the bunker.
