Look how disappointed Cas is! He knows Dean is mad at him, he knows Dean is a jerk when he’s angry, that he always close himself and bury his feelings and he only let out anger and frustration, but Sam? Sam is his ally, his friend, the rational one, the calm one, the one who always is willing to consider what he has to say and the one who think straight. But this time, Cas thought he was having some kind of support from his brother, the first dad to his son, the one who was there when Jack died on that bed, who could understand the grief he was feeling in that moment… but no. Sam was in fight or flight mode and needed to think a way to get out of there, so he agreed with his brother in this. Cas is brokenhearted, if someone could understand him is Sam, but when Sam decided to agree with Dean, Cas realized he was alone, and his pain won’t be adressed any time soon.