There's just something so incredibly innocent about Castiel when he first goes on a mission with Dean, especially when Dean has to turn Castiel's badge ride side up because Cas has it done all wrong.
Dean just blurting out there is no singing in Supernatural easily sets the stage for the rest of the 200th episode that is so very special in its own right.
Why do they call this place The Empty? It's full. It's full of sorrow and despair playing over and over again of angels and demons dreaming about their regrets. Forever.