Thank you. Really, thank you. I won't let you down.
their relationship is so uneven at this moment. there is unequal footing between them now because sam went down the dark path and 'betrayed' dean (choosing a demon over dean: ruby). even though sam had been fighting for more equal footing, dean is now even higher up in their relationship because sam needs to prove himself. sam's incredibly thankful that dean is giving him a second chance, but when do they get on equal footing again? when sam goes to hell? but then he comes back without a soul and does some terrible things, and dean saves his soul. still unequal footing. (dean does kill amy in s7 which kind of is his first betrayal against sam - that sam actually counts and stands up for. dean feels incredibly guilty for that but there's no apology and sam ends up agreeing with him). anyways, in s8 sam has let dean down by not looking for him when he was in purgatory, and dean keeps beating him up over it. sam doesn't feel he's enough or worth his brother's admiration or love ("you think i screw up everything i try"). so they're still not on equal footing, sam is the one who 'is lesser'
Do you know what I confessed in there? What my greatest sin was?
maybe the change starts from s9 - s10 when dean betrays him by letting an angel in. and then dean is on a dark path of his own with the MoC, and this time sam has to pull him away from it. now dean has 'betrayed' him (choosing a demon over sam: crowley) and has done horrible stuff as well. now they've both messed up and let the other one down. (but sam doesn't blame dean for the mark of cain, nor does he blame him for everything he did while he was a demon, like trying to kill him).
dean knows how far off he went (asking if sam wants a divorce and acknowledging that he tried to kill his own brother). but he also points out later that sam did cross some lines in trying to save him ("go dark"). either way, for dean this might be the turning point where he sees himself also as someone who is in the wrong, and not just sam. but sam still keeps dean to a high standard.
maybe it's just younger brother devotion. to sam, dean is nothing but good while sam himself is flawed - more flawed than he believes dean could ever be
"You will never ever hear me say, that you - the real you - is anything but good"