The storming out was expected, the reaction from Castiel was less so.
"...She's not wrong. I don't care about Sam? Maybe not in a particularly good way, and I get that, I do. But telling me I don't. Never did maybe. That's bull, and I don't have to take it from her when even Dean is being less hostile about it all. I know what I did. I know how much danger I put Sam in. I know how far I pushed him toward becoming that. I really do. And I did apologise to her, and to Dean. Meant it too. So don't tell me I'm not trying"
She closed her eyes turning away from him furiously, but trying to calm herself.
"...If you don't believe me, if you don't...at least try to, then why am I doing this. You're pretty much the reason. Or a very big part of it. Should I open a vein go and find Sam and let them wipe me from existance? Would that be easier for you? What am I doing here Cas? Just...tell me would you because me, I thought I was trying?"