There were very few subjects which Castiel knew more about than Dean Winchester. In Hell he had seen him literally stripped bare to his soul, not even his body to use as a shield. On Earth, he saw the world as Dean did every time their gazes locked. The world through Dean's eyes was a fascinating place. There was a lot of black, a little white and almost no gray.
Before he had set foot on earth for the first time in 2000 years, Castiel and every angel in his garrison had been informed that the battle to evil was being lost as one human after another was surrendering their soul to Lucifer. And yet Dean believed that humans were almost all universally innocent, naive, in need of protection from the evil that lurked in the world, in spite of their multitude of sins. Including Sam. Especially Sam.
Like all of the angels, Castiel had harbored no affection for Sam Winchester. When the garrison was readying for the assault on Hell to rescue Dean, they had been warned about Sam—told that he had been handpicked for some unknown dark fate by one of the highest ranking demons in Lucifer's army and that he was beyond redemption no matter how much it might appear otherwise. They were warned to keep their distance and their eyes open when it came to Sam.
So as interesting as Castiel found the world as viewed by Dean, it was Sam as seen through Dean's eyes that had been the most conflicting. Before he even met Sam he found himself softening toward the younger Winchester. Dean's faith in Sam was such a core part of who Dean was that to doubt Sam was to doubt Dean. And although Castiel questioned everything else he had ever believed in, Castiel's trust in Dean Winchester was absolute.
While Dean's faith in humanity had inspired Castiel to question the increasingly disturbing orders he had been receiving, it was Dean's certainty that Sam was worth saving—when in Dean's mind he himself was not—that allowed Castiel to finally understand the true nature of love and make the decision to rebel.