Why? Why did Cas’s grace have to be hidden in Don Quixote? Of course, I know this is how Metatron sees Castiel – he said as much in the episode, angels have “delusions of grandeur”. He sees Castiel as being the most delusional of all– claiming he is on a divine quest, doing countless foolish things that ultimately end with him hurting himself. He attacks windmills, saying they’re monsters. And even though, unlike Don Quixote, Castiel won’t flat out admit that all he has done, is doing and ever will do is in the name of Dean … his Dulcinea, Metatron knows that is the reasoning. That is why he wanted Castiel to say it– just admit why he is really risking himself in so many ways. The corrupted, former scribe sees the irony in Castiel’s story … the angel is tearing himself apart for a man who he sees as noble and worthy, but is truly nothing more than a peasant.
In this tale that he is writing, Metatron has made himself Cervantes, the nobleman determined in exposing the foolish knight’s logic. He doesn’t want Castiel to die, he just wants the angel to admit that he has no real cause. He wants to make him admit that underneath the wings and the grace, he really is just as flawed as the humans. He wants to leave Cas beaten and bloody in the dirt– dominated like Don Quixote was, with no other option than to say that he is not noble. His Dulcinea is not a princess, not a beauty worth the glory he gave her name. Metatron wants Castiel to be the last of the chivalrous knights of heaven; and when the angel finally does meet his end– his story will be tragic, but not because he lost his love and not because he lost his fight … but because he was a fool. and never had a true love to fight for in the first place.