Can you honestly imagine what Sam has been going through all of these years, thinking that he disappointed Dean at every turn?
When he died and Dean sold his soul for him, he probably felt so guilty. He no doubt thought that Dean was disappointed in him and had to throw himself into hell to undo something Sam should have been able to prevent.
Then Dean was being torn to shreds in front of him by the Hellhounds and Sam could physically do nothing. He should have found a way to save Dean - but instead he failed Dean once more.
And when he tried so desperately to do the right thing with the demon blood and Dean told him that he was wrong, that he was a monster, can you just imagine the way Sam felt, hearing Dean actually tell him those things? All Sam wanted was for Dean to support him, to believe in him, to trust him, but he just disappointed him.
How about when Sam let Lucifer in, against Dean’s approval. Don’t you think Sam was just screaming from inside his own body, desperate to gain control so that he could make Dean proud?
Or when Dean came back from Purgatory with a vampire as his trusted sidekick. How do you think Sam felt, knowing that Dean trusted that monster over him? Dean had been so disappointed by Sam’s failures that he chose someone else, Sam thought.
But worst of all, no matter how big or small, everything Sam did, he assumed that Dean was disappointed by him. He could always do better. He could try harder. Until he physically couldn’t anymore and then he decided it would be better to die than to keep struggling for his brother’s approval.
And all along, Dean trusted Sammy above anyone else in the entire freaking universe.