Guilt was his weakness, he knew that, and so did everyone else. His brother had been used against him several times. He had always regretted bringing Sam back into this life. The worst part of it? He had done so many things to his younger brother. He just had to face the possibility that Sam was right, he just didn't want to be left alone, maybe he did more bad than he did good. No matter how hard he tried, his world always crashed around him, the people he loved always got hurt.
Yes, he had suffered for those mistakes, but it wasn't enough. 40 years in Hell, a year in Purgatory and he was still here, still screwing up Sammy's life. It was times like this that Dean hated the most. He was currently sat in his room inside the bunker, he was alone and that's what made it worse, he had time to think. To reflect over everything, his pain, suffering and most of all...his guilt. He felt so much of it. If something happens, he feels responsible for the people that got hurt and for what went wrong.
If it wasn't for you Sammy wouldn't even be here.
There has always been a war raging inside him, a battle of survival and emotions.
In fact, you wouldn't even be here if it wasn't for Dad, that was a mistake that he paid for. He suffered for you, that's your fault. He made a deal, he sacrificed his soul and life for yours.
The guilt, the depression and loneliness, it was all pushed away by his constant want for alcohol. Sex was pointless to him now but it drowned them out, only for a while but that's all he needed in order to keep going.
The life of a hunter is all he really knew. The pain was just a part of that life style. The benefits of caring for another are few, especially in the 'Winchester's' world. To feel the lose of friends and family, to then feel the anger and the surge of revenge pulse through your veins. It was stupid to care for someone too deeply, to even love them was a mistake it only hurt more when they left. Everyone leaves at some point, it's just a matter of when and how they would go.
It used to be easy. Hey, it used to be fun. He woke every morning knowing he saved more people than he killed, he knew he was eliminating a tiny fraction of evil from the worlds roots every day by just doing his job, what he was raised to do. It was all forced on him, hunting, survival...From the moment of her death he was condemned by his father to follow in his footsteps. he didn't have a choice but it was worth it wasn't it? To save others?
The guilt used to build on him, crushing him under it's weight. Not physically, but emotionally it hurt him, tore at his sanity. His innocence had been snatched from him when he learned of the true evil, of the creatures that lurked in the dark and even out on the public streets. He was raised to fight and defend, the responsibility of raising his little brother had forced him to be mature, skipping past most of his childhood.
Things have changed. He was liability to himself. He had become less rational, his senses were sharper and he was more cunning and deceitful in their hunts. The Demon's plans for Sam years ago had been nothing but a small bump in the road compared to what they faced today.
The thought of Sam, his brother, was all that got him up in the morning, it's all that kept him fighting for his life each day. That and the pleasure of knowing he would soon find Abaddon, pierce her skin with the First Blade.
Then he was going to find Gadreel, avenge Kevin's death. Still the guilt was placed on him for Kevin and the many others that had perished when becoming close to him. He knew he was poison, a danger to those around him but still, if he was alive there was bound to be casualties.
He may be condemned to a life full of pain, lose an confusion but he could keep fighting or cower in the enemies footsteps. He chose option One. No matter how much it hurt him, he wasn't raised to back down from a fight and he never would.