After all that had happened, the last thing Castiel had hoped to see in Dean was any kind of faith. Be it in him, any other angel or Heaven overall.
That's why he found it hard to explain his pleasure upon Dean actually praying for him to help him, praying as if he actually meant it – and maybe he did. It was impossible not to react to the call.
And maybe that was also the reason that Castiel, even though he knew it was not his place to help the Winchesters at this point, gave Dean the one hint that he needed to ensure Sam's safety – even if it meant risking to get in trouble for endangering the prophet.
Because what did this prophet's life mean to him, personally, compared to Dean's trust?

Maybe he should tell Dean at some point, Castiel figured. About everything that was going on. Heaven's plan – at least as much as he knew about it – the necessary sacrifices that needed to be made, and also about how intense his doubts had actually become in the short time he had spent on earth.
That was why he visited Dean in one of his dreams. What he didn't know though, was the punishment he would face for even just trying.

Being torn back to Heaven had taught Castiel many things, and the way Jimmy Novak tried to keep him out only added to that. Castiel wanted to believe that people had faith. In the angels, in God and in the Greater Good. But all he had learned from the past was that humans believed for only as long as they needed help.
They easily took, but hardly ever gave anything back. Was that really worth fighting for? This whole place was full of hatred, pain and destruction – nothing down here was desirable, and more than anything, Castiel just wanted to leave earth as soon as he could.
You're lying to yourself, was what his mind told him. And you know it.
Was he being brainwashed by the angels? Was seeing what was going on in Heaven, and seeing what happened down on earth, just manipulating him to look for something to fight for? Was he too weak to make his own decisions?
No, was what he wanted to think. It was his duty as a servant of God to do exactly that: Serve God.
But why, if that was his destiny, did it feel so wrong?

Castiel had expected Dean to forget about him and pray to someone, maybe something else for help. After all, the angel had made it very clear that he only served Heaven, neither humanity nor Dean.
Yet, the pleads for him to help the Winchesters were so crystal clear, so honest and desperate, that he found himself unable to ignore them.

"Sam could kill Lilith," he explained upon being asked. "But the amount of demon blood needed for it would make him inhumane."

That was the truth, even though part of the reason he put so much emphasis on that was because Dean cared about Sam.
Was Castiel trying to manipulating Dean? He didn't know, honestly, and maybe he didn't want to. And it probably didn't matter, because at the end of it all, he, who had tried to assure Dean that Sam could be saved, could be cured, was the one to set the latter free and make way for him to find and kill Lilith.
Because that was the order Heaven had placed on him, wasn't it?

There was no room for personal issues, for feelings that the angels weren't even supposed to have. The only thing that mattered was bringing peace to the world, even if it meant having to accept sacrifices in the course of it.
Heaven will have mercy, was all that Castiel could think. When this is all over, things will finally start making sense.
He tried to believe, as it was all he could do.
Rebelling hadn't done Anna any good, and he wasn't going to fall for something as finite as humanity like she had.
Maybe in a hundred years, he would have already forgotten about all this. No one would question his loyalty.