This moment is generally played off as humorous, but I think it actually says a lot about their relationship. This isn't the sort of thing that Castiel would ever say to Dean, because he knows that Dean still holds it, and every other thing Castiel has done that Dean didn't like, against him. Whereas Castiel brings up what he did to Sam without any reservations due to simple indignation that Sam accused him of being a bad liar, because he knows without even having to think about it that Sam won't get upset. Sam was the only one who still believed in Castiel at the beginning of s7, despite the fact that of him, Dean, and Bobby, Castiel inflicted the most harm upon Sam. Castiel feels safe with Sam, so safe that he'll bring up the worst thing he ever did to Sam and know that it'll be alright.
Their relationship is one of the purest things in Supernatural I can't believe it took me until s13 to see it.
I see this also in that little moment in Stairway to Heaven (I think?) When newly human Cas tells Sam to step aside so he can break down the door? His little "Step aside, I got this." Followed by his embarrassed "I don't got this." when he can't.
I always like that moment because it's Castiel at his weakest and most vulnerable and yet he still feels safe enough to joke around with Sam because he knows Sam won't tease him or hold it over him. Cas is letting his shields down and being playful because he trusts Sam and feels safe with him.
Anyway I very much endorse op, thank you
I actually wrote a whole post about that moment! Castiel never allows himself to be silly in front of anyone except Sam. It reminds me of this moment:
Sam is playful with him, and Castiel has no compunction about reciprocating, because he trusts Sam. He allows himself to behave in ways he never does at other times, and it's one of the many reasons I love Sastiel.
