In a movie or story which ends a chain of catastrophes with the twist that it was all a dream certain formal constraints must be adhered to. When looking back you notice numerous 'tells' that revealed the dream nature of the presented world. You didn't notice these on the way in because you were expecting a pay-off in pity, fear or vicarious enjoyment, and so had no wish to un-suspend disbelief.. The narrative has to justify its facile ending by making your complicity its real subject.