He feels so bad that he wasn't strong enough to tear open the gates of Hell himself, that his soul wasn't valuable enough to trade for Dean's, that he just wasn't good enough. He can't even hold eye contact with Dean while he says it, and Sam's the type to hold your gaze and never turn away. But he spent four months drowning in self-loathing and blood and vengeance and suicidal tactics because he was powerless to save Dean, and someone else came in and did it for him, so he failed. Again.
I wasn't sure where to pin this one so I'll just pin it on Matthew's Board. :) Dr.SuperWhoLock! In Order: Dr. Who, Dr. Spencer Reid (Criminal Minds), Sam, Cass, Dean (Supernatural) Sherlock and Watson (BBC Sherlock)