It just goes to show how devastated Dean was throughout most of Season 9, thanks to Sam's hurtful words. This is soon after Sam said he wouldn't save Dean's life. Yes, he meant in the way Dean saved Sam's life in the season opener, but we all know that's not how Dean heard it (and neither did many of us). Sam didn't even want to work together as brothers, but keep their partnership strictly business. So Dean's got to feel that he risked everything, trusted an angel (and was deceived), and got his brother back only to have Sam reject him as a sibling and admit he'd let him die. What's the point of fighting to stay alive?
Dean has often been ready to die, seeing himself as a grunt back in S8 and ready to do the trials. You could go back to Dean accepting the Mark of Cain so easily as another type of death wish. He was easily talked into the mission to get the First Blade by Crowley and was so hellbent on killing Abaddon that he was willing to risk everything (and not even question the consequences) to get the Mark. Even by season's end he admitted he was ready to sacrifice himself in order to kill her.
Dean: I'm gonna take my shot, for better or worse.
Sam: I know.
Dean: No matter the consequences.
Of course, this is not unusual behavior for Dean, but his motivation this season was spurred on by Sam's resentment of Dean's actions in saving his life. And let's not get into who was right and wrong - they both were. This little moment just struck me as so sad. My poor Dean. Those dead eyes say so much (as they always do). Dean is done.