"You've noticed, too." Cas seemed to breathe a sigh of relief, bowing his head. "I don't know. I can't explain it. Something's happened to him in the time I last saw him and now. I don't know how long it's been, but there's something he's not telling me. Or you."
Dean was likely to keep things from Castiel, even though sometimes he felt he knew Dean's every secret. For years, Dean prayed nearly every night, talking into the silence and using Cas's name — and Cas always heard it, even if he didn't or couldn't answer. He knew Dean's fears, he knew his worries, he knew the little things he was thinking about but didn't want to say to anyone in person.