Dean felt his resolve wobble slightly, but he wasn't going to let Sam's argument of 'you woulda done the same' stop him from getting the truth. He couldn't help feel like there was more, even though he desperately wished there wasn't. "If I had kept it from you and was freakin obvious about lying, you would be right where I am demanding answers," turning that same argument back on him. His words about Cas sting more than Dean expected, though, pissed and hurt at being lied to but still breaking at the thought of the angel, their angel, dying for a cause Dean had dragged him into.