I mean, why the hell can't he take responsibility for his role in how things played out?
Well, Dean did take responsibility once, namely back in Fallen Idols. But afterwards it was never mentioned again, not by Dean, nor by anyone else, whenever the topic came up. It bugs me to no end that nowadays Dean not only does not disabuse Sam (or others) of the notion that Sam is solely responsible for Lucifer's release, but also accuses him of the deed himself (Sacrifice). S8 has systematically undermined Dean's understanding of Sam as a victim, both in the matter of Lucifer's release as well as in the matter of his soullessness. It drives me nuts. :(