Castiel had always been the most faithful angel in heaven. Blind obedience, loyal to an absent god. He never ruffled feathers, or broke rules. There were a lot of rules in heaven, don't ask questions being the biggest. Another major rule was to never seek out your parent.
God was technically Castiel's grandfather, he was born from the grace of an archangel. Angel's birthed the lower angels and so on.
When Balthazar told Castiel that he found out Gabriel was his father he wasn't surprised. Balthazar's pale yellow wings were a muted version of Gabriel's gold ones. Castiel also knew that Samandriel was Michael's son, because of how he doted on the angel. (It was for this exact reason that angel's were not supposed to know.)
He tried not to think about who could be his parent. There was Michael, Lucifer, Gabriel, Uriel, Raphael, Selaphiel and Annaiel.
The Archangels always treated Castiel as if he was fragile. The lower angels always mocked his black plumes. That was why he was so eager to prove himself, when it came to saving the righteous man.
Castiel had descended to hell on his own, the thick flames licked at his wings. The screaming of a million souls, all of which reached out for him, caused his head to spin. There was only one soul that shined as brightly as the one of the righteous man and once he had that precious soul in his grip he retreated. He held tightly to his beautiful prize, dragging the soul out of the pit, and placing it back into the healed body.
That had been almost two years ago.
Castiel found in those two years that the righteous man would not only help him discover his greatest strength but also his be biggest downfall. Castiel had almost been thrown from heaven due to his sympathies for Dean Winchester. Sam Winchester too. However, when the other angels scolded him it always came down to Dean.
Dean who had taught Castiel about doubt, and about breaking the rules. Dean, who Castiel found so important that he hid him (and Sam) away from heaven and hell. Dean who introduced him to all the wonders humanity had to offer, compassion, hamburgers, and music.
Castiel could think for days about all the wonders that were Dean Winchester.
He had such a strong emotion for the righteous man. One he didn't fully understand. Balthazar had told him that it was love, but Castiel didn't truly understand that either. All he knew is that Dean Winchester was good man, who spelled like leather and gunpowder, who Castiel would give defy all orders for.
That is why Castiel would be going with the Winchesters, Ellen and Jo to kill the devil.