Castiel had been a faithful servant of the lord since his creation. When the lord named him the angel of Thursday, Castiel had taken his duty seriously. Every thursday he would descend to Earth and guard humanity, and in return he would watch them. They never stopped surprising him, the humans. Castiel could understand the need to celebrate the day one was born, for life was God's greatest gift. As he could see the importance of remember great military victories, as triumph over evil was to always be celebrated, but the humans celebrated the strangest things. Castiel once watched a crowd of humans cheer as one gutted another inside a stadium. He understood that they were fighting, but couldn't comprehend how bloodshed was worthy entertainment. He had been in battles far too often to find joy in such an activity.
Castiel had never been among them, for all that he had watched. He had not had the opportunity until he was told to guard Dean Winchester. And, among the incomprehensible mass that was humanity, Castiel found that Dean was even more of a mystery to him. He said the strangest things all the time and would look at Castiel like he was the crazy one for not understanding his gibberish. He would later be informed of television and novels and would come to understand the "lingo" Dean always spouted, but in those early days Castiel was often left a little lost when he talked to Dean. He didn't understand why it filled him with excitement.
When Castiel fell, he felt lost, different and deeper, and turned to Dean Winchester for answers. How a human could educate an angel, Castiel did not know, but he didn't know what to do and so he turned to the only other that could help. And Dean taught Castiel much of free will and choice and Castiel, cast away and alone save for a handful of humans, had never felt more powerful, more free. He knew that he had not gained the strength of an archangel or that he was suddenly different. But he felt different. So when he faced certain death, Castiel was not afraid.
Castiel did not expect to come back. To come back, time and time again. Was this a blessing from his father? Or a punishment? Castiel thought at first it might be the former, but as he amassed strength to fight Raphael and worked with Crowley, a part of his subconscious, and it was not Jimmy for he was already in heaven, knew it was the latter. Despite that niggling in the back of his mind, Castiel paved onward in a frantic attempt to help, to fix. He wanted to make things right. To help his brothers and sisters. To be like Dean. To be the man- angel -that Dean thought he was. Dean knew something was wrong, but Castiel didn't want to worry him. The faith Dean had in him drove him forward, even if that small part of his mind ached in remorse and guilt.
When Castiel snapped back to himself, when he realized what he'd done, he wanted to apologize to Dean. Dean who was looking so betrayed, who wouldn't come any closer to the angel who went behind his back, killed thousands, and lied to Dean's face every step of the way. A friend who had hurt his little brother and worked with a demon. Castiel wanted to apologize, but he was out of time. He managed one word before he faded away. Leviathans.
Still Castiel was cursed to not remain dead. He was cursed to keep coming back, a physical reminder of all the wrong he had done. And Castiel loved Dean. Loved Sam. Loved Bobby. He loved them, his family by choice. But Castiel believed he had lost the right to be family a long time ago. Dean wouldn't hear it, and when he messed up again, tricked by Metatron, Dean was there to set him right. To chastise him and comfort him in the same breath.
Castiel felt he didn't deserve Dean, but in his short stout as a human, Castiel had picked up their sins, and Castiel was too selfish to let go.