Dean was never one to shun responsibilities. He couldn't when he was young, and didn't know how when he grew older. He was there for Sam since Sam was six months old, and yet Sam left him many times. Dean was there for John since Dean was four years old, and yet John left him.
Dean remembered his mom many times, but mostly he remembered the last sound he heard from his mom. It wasn't the sweet lullaby she sang when she put him to bed; it was the last scream she cried before she died. That scream is what wakes Dean up in the middle of the night, even now, 30 years after she died.
It takes a special man to be able to see what is out there, to hunt it down, risking his life over and over to save others and Dean has done this since he was a child. At 16, when he had his first big kill, he decided to dedicate his life to hunting. As soon as Sam could go to school Dean hunted with his father. When Sam left for college, Dean hunted alone most times.
The angels had an agenda; to use the righteous man to bring on the Apocalypse, only this time with the finale they felt cheated out of the first time. They used Dean, the righteous man, the first in thousands of years, for this goal. Their plan went awry because Dean, in his love for mankind and for his world, refused to play the part that would destroy it. Even as Sam said yes, to play his part, to give into his weaknesses, Dean still remained strong. Playing the hero wasn't worth the destruction of the world, and so Dean stood back realizing that Michael, without having his strong vessel, his true vessel, could not destroy the world and Lucifer, once distracted by Dean's audacity to stand up to him, could be defeated. After all, the most evil of all evils still had the weakness of pride and arrogance, and that was Lucifer's fall.
Dean Winchester has always been the quiet man, the subtle hero that saves others for the sake of saving, while putting his own needs and desires on hold. It happened with Lisa, when he became so close to her and Ben that he put them in danger. He left them, quietly and without memories of Dean Winchester. It broke his heart, and still does. He left Cassie, knowing that his job was more important, and it broke his heart. It still does. But Dean is a man who knows that if he didn't do his job, innocents would die. Innocents like Lucas and other children. Like Haley and her brothers, innocents who felt guilty like Charlie. So many others.
Dean Winchester will always be unique among men, and Cas realizes this, even if Sam never did, nor did John. Bobby had a hint of it, but it was too complicated for even someone as experienced and knowledgeable as Bobby. Jo Harvelle sensed it, and she fell in love with him. Ellen realized it, and encouraged him. Rufus knew. He knew by that first time they talked, but Rufus kept it to himself.
Cas does see Dean, really sees him. He did from the moment he plucked him out of hell, and it was this knowing that created the bond in which Cas has never been able to walk away from. Dean Winchester doesn't know, nobody told him, but Cas knows.
