Has this been done before? It must have. Really.
Guys I am sorry about this. I am taking a plane tomorrow and heading off to an unknown country for a YEAR and I am beyond terrified and yes this is what came out of it. God I don't even KNOW. Cas please forgive me.
Since Castiel had fallen, he had been learning about the throes of humanity, the hard way.
Day after day, his newly acquired mortality made itself known to him, in the very worse ways one could think of. Pain and exhaustion and needs and cravings. Feeling drained, feeling sore, feeling empty, feeling small, feeling and feeling until his head seemed crowded, felt heavy.
He learned that one's body was not a mere instrument, a mere tool for transportation.
His body was becoming him, was merging with him and he could no longer ignore nor neglect it.
That one evening, as he returned to their motel in a hurry, Dean and Sam on his tail, pain coursing through his body, Castiel understood just that. He could not inflict anything on his body and expect no consequence. He could not merely push it to the edge, then become surprised as it came out bruised and damaged, beyond fixing.
He also learned that sometimes... even a friend could not be trusted. Even the dearest of all. Even the one for whom he had sacrificed everything. Even the one he held so close to his heart.
Life as a human being was painful and humiliating, Castiel thought as he eventually stood, then winced as he remembered Dean's instructions, and reached to his right for a scrap of paper.
As he exited the bathroom and stepped back into their room, slowly raising his eyes to the Winchester brothers seated on a bed, he looked sad and miserable. He felt pathetic, hurt, and... And dirty. When he did see Dean and Sam's faces however, he just stood there, shell shocked, because where he expected compassion and sympathy for the ordeal he had just gone through, all he received in return was two looks of suppressed laughter and incredibly irritating mirth. Dean was trying so bad to hold back that there were tears in his eyes. Through his distress, Castiel failed to see whatit was that could be considered funny about all this.
He remembered Dean's words from earlier this evening, and felt betrayed, as though he had been tricked.
"Hey, Cas, tonight's Mexican food. Trust me buddy, you gonna love it!"
Human bodies were terrifying, and full of lies, that Castiel was sure of.