I have the headcanon that if Dean stumbled across the Mirror of Erised, he wouldn’t see his reflection or any kind of perfect life for himself.
Sam would be there, yes, and Cas, both of them looking happy and content. But there would be an empty space where Dean should be.
Because even in Dean’s ‘dream life’ provided by the djinn, he was an alcoholic and him and Sam hardly spoke because he was no good. Dean wants the people he loves to be happy. But he also is determined to believe that they would be happiest if he wasn’t there. So his deepest desire is that Sam and Cas could be happy, without him being around to screw things up by being the other vessel that kicks off the Apocalypse or by persuading Cas to leave Heaven.