Castiel had never dreamed that he would fall. He had always been supremely obedient, following the orders, when they came, from his superiors; praying to his Father, doing his duties. He was good; loyal. And then he was ordered to save Dean Winchester.
His thoughts changed after that; his perception altered. He found that he no longer believed that the angels were pure, perfect beings of celestial grace. Castiel preferred to spend time with Dean and Sam, even when he normally didn't understand a lot of their chosen conversation topics.
Yes, Sam was the boy king and Castiel was uneasy around him, but he was kind and loyal to his family. Yes, Dean drank and swore just a little too much, showed affection too little; but he was a moral man. Castiel grew to love both of his charges; in that sweet way that you love a sibling.
And then Castiel was pulled back into Heaven. After that he was bitter. Even Angels hold grudges; although the Winchester brothers were not to be held responsible for Castiel forgetting his place. He tried to be stern and mean to them after that, hoping they would stop calling on him. But instead they seemed to rely on him even more.
He loves Dean a different way afther that.
That is why he falls.
For him.