Castiel was very aware of how obsessed Sam could get. It was something he was deeply afraid of and an incredibly delicate path to tread.
"And if he never understands?" He looked to Ruby with a faint frown. "He's human, Ruby. He may never understand why your faith was so important to you. He may only see the Lucifer that he understands. What will you do then? He may never see. Are you prepared for that possibility?" He knew, he knew full well that Sam wouldn't be the only one getting obsessed and the pair of them, obsessed with each other again, that could only go badly.
"The first war," he started, awkwardly. "To most people it was no doubt this glorious thing, demons in the Pit probably see it one way. But to us, to the angels, it was watching our brothers tear each other apart. Lucifer is my brother and I still love him. Even after everything he did, that won't ever change. I just find it hard to talk about him the way others see him."