The only thing holding Dean Winchester's soul in his body is the smallest bit of Castiel's grace. Sam only finds this out when Cas is killed by Raphael. In one, bright, rapid flash Cas dies, wings burnt out into the dry earth of some nameless field. Many miles away, Sam has one second of Dean's shocked face, before his expression goes perfectly blank and he collapses. When the body rises it's a soulless automaton wearing his brother's skin.
In hell, Dean's soul wakes on the rack, screaming and screaming and screaming, but there is no one left to save him.