Joshua is kind and sympathetic, and is painfully aware how Dean is losing faith. He’s apologetic over the fact he can’t tell them what they hoped to hear. He sends them back to Earth this time with their memories and once Sam and Dean awake to the bloody mess they left behind, their perspectives couldn’t be more opposite. Dean calls Castiel and tells him of the conversation, and that’s enough for Castiel to lose all hope. He somberly returns the amulet to Dean, calling it worthless, and flies off. Sam is determined to find another way and tries to encourage Dean in their fight, but Dean follows Castiel’s lead and quietly gives up. He even drops his precious amulet in the trash on the way out, leaving Sam alone to ponder their
I usually read some fan reactions before writing a review and sad to say, I think a few missed what's happening here. The idea of God turning his back on man is nothing new. This is straight out of Paradise Lost (published in 1667), where God denounces responsibility for man because of man’s choice to use free will. He will not intervene or help. It’s up to man to solve his own problems. It’s meant to be a test of character, a test of faith, a test of the resolve of humanity.
Joshua even tells Sam and Dean “Why does he allow evil in the first place? You could drive yourself nuts asking questions like that.” He’s trying to get them to focus on the bigger picture, which is fighting the apocalypse. I don’t think Joshua is implying they should give up. He even tells them he’s rooting for them and feels bad he couldn’t deliver the answer they wanted. He points out that they were saved by being put on the plane and granted them salvation in Heaven, despite all that they’ve done. They are in God’s good graces.
The issue with Dean is he’s not in a good place right now and can’t find the way out. He’s at wits end due to the impossible burdens he’s put on himself to save everyone in the world alone. He needs help and isn’t getting the kind he wants and that’s putting him into the lowest of lows. Finding God to him is truly the last act of a desperate man. Once that didn’t turn out the way he hoped, his spirit is broken. They’ve been building up to this fall for a while now, at least since “Abandon All Hope.” When Dean throws the amulet away, I don’t think it is a slam on Sam so much as the action of a man who has lost faith in everything. Family, God, the real world, people; all that he held dear. It doesn’t mean anything to him anymore because nothing means anything to him anymore. He believes Castiel, it’s worthless.