It's an impossible task.

Sam can't tell Dean. How can you protest someone loving you or complain about someone caring too much? Dean gives up so much for him, has already given up so much for him; Sam can't throw that back in his face, not when he knows that any attempt at normalcy he has ever known is due exclusively to Dean's sacrifices. Sam is first between them, has always been first, separated by the sheer height of the pedestal Dean puts him on.

And that's the problem. He can stay up for days in a row, will happily give up hours to thankless research, will move whatever he can (because he can't move heaven and earth, Dean's the one who thinks that way) to save his brother, to protect the only family he's got left now. It is easier to feel unappreciated, easier to be irritated at his brother, easier to cling to some vestige of ordinary even in the form of sibling squabbling, than it is to see the way Dean looks at him sometimes. It's too easy to see the calculation behind his eyes, the weighing and measuring that always comes up Sam, even when it's a choice between Dean's life and Sam's.

Every time Sam makes a mistake he feels the weight of disappointment even before Dean looks at him that way. With John it was easier; disappointment was just fatherly disapproval, normal and human. With Dean it's like he's pulling the world away from him brother, like he's some god who isn't allowed to be fallible for one moment. And even the proof of his humanity isn't enough to change anything. Dean shakes off the disappointment and the evidence that Sam is just a person, or maybe puts himself back in a position of less-than a person, and the pedestal is back in place and the pressure is back on.

When Dean sells his soul for Sam's life, Sam knows, right then, that he can never make up for the things Dean has done for him. He suspected, before, but he kept trying, kept hoping. It's like a door slamming shut, a condemnation to fail before he's even had a chance to try. Who can live up to the worth of two people when being just one is hard enough?

Sam has given up even before Dean is dragged down to hell and he is left alone. And when Ruby shows up, and offers a smile and strength and power, he knows it won't be enough, it'll never be enough to make up for Dean's life and soul and love, all the things his brother has sacrificed for him. And he knows Dean would be disappointed in him, that he's letting his brother down again, even now, when Dean's not even here to silently judge him.

He takes the offer anyway, or maybe because he knows he's letting Dean down. He's going to disappoint anyway. At least this way he's doing it deliberately.