I personally think the reason that Castiel sent Dean back is because Dean going back is exactly how we got canon as we know it.
Seriously. If Dean hadn't gone back Mary and Samuel never would've gone to Liddy's, Samuel wouldn't've been possessed, and YED wouldn't've gotten his eye on Mary. He wouldn't've killed John, Mary wouldn't've made the deal and Sammy wouldn't be a Special Little Boy.
This is why you can't change your past. Because either anything you do you've already done or you end up negating your actions and rendering it all unchanged. Like. Dean couldn't save Mary from dying, because if she lived and he wasn't a hunter he never would've gone back and warned her, therefor she would've died.
And as for YED saying he can't bring Dean back, but he knows someone who can? He's a demon. They tend to lie, sometimes. And if he was telling the truth? See the comment he made about the red tape. He can't make a Special Kid without being invited so maybe he can't give back a life he didn't directly take.
In other news, Yussie totally just finished watching Ghostfacers on his own. :D
TEN DAYS UNTIL WINCON, HMOG.