Dean apologizing was... well it surprised Sam. Disarmed him, in spite of his automatic impulse to enclose himself in and emotional buttress against self loathing, disappointment and loss.
He glanced at Dean and swallowed. "Don't..." Teeth closed over his lower lip, stopping the rest of that sentence. After a second or so he finished, "...you don't need to apologize to me. Ever."
It was too much, Sam knew it, but the words came out because he really did believe them.
They both got by sometimes pretending things were 'normal', trying to act as if none of the horrific events of their lives had really happened, or had mattered. But that was a game Sam had less and less strength for.
"You're not dumb. That's one thing you never were, and never will be."
Sure, Dean acted like a buffoon at times but Sam understood why - it was how he coped.