I can’t remember if we’ve ever discussed this, but one thing that really jumped out at me while re-watching Stairway to Heaven just now was how Cas kept glancing over at Dean as Metatron spoke. In the top gif, Metatron has just implied that none of them know the real Castiel, and it’s clear why he looked at Dean then, as he had been worrying the entire episode that Dean didn’t trust him.
The second gif, however, is what really breaks my heart. Metatron has just brought up Cas’ stolen grace, and how it’s burning him out. Up to this point Dean had no idea that Cas was dying, and I think he wanted to keep this information from him from this at all costs. Cas was in the midsts of trying to get to Heaven in order to try and save Dean from the mark, all the while trying to protect him from the knowledge that he was likely not going to be around much longer. I think Cas felt guilt for not telling him for sure, but most of all he felt protective. To quote Karen Singer, when Dean asked her in 5x15 why she didn’t just tell Bobby that she remembered how she died:
My job is to bring him peace…not pain.
And recall how Karen told Dean he couldn’t understand this because he had never been in love. Then recall that moments later Metatron reveals Castiel’s true weakness. But, like Karen, it was not humanity he was protecting, it was always one man…
Agree with your thinking here.