I took what Castiel said at some point in the finale of season 4 and kinda gave back up thoughts to it…I don't really like this one but…here goes
Castiel liked Sam. He knew Sam knew he knew what Sam'd been doing the past year. He also knew Sam felt uneasy around him. But before all of this they'd been building a bit of a...relationship, that's what Oprah called it anyway. Lucifer would probably kill Sam tonight, that made Cas sad, agonizingly so. Which surprised him, sadness…was a human emotion. And after the guilt trip this afternoon when Dean'd been brought in caged like a tiger in a zoo it hit him. The cold hard unavoidable truth, he was fated to Fall.