S2 E22 - All Hell Breaks Loose: Part 2
Dean talking to dead Sam (links to YouTube)
Dean: "I was trying to protect you, keep you safe. Dad didn't even have to tell me, it was just always my responsibility. It was like I had one job. I had one job and I screwed it up. I blew it and for that I am sorry. I guess that's what I do, I let down the people I love. I let dad down. And now, I guess I'm supposed to let you down, too. How can I? How am I supposed to live with that? What am I supposed to do, Sammy?"
Breaks my heart, every fucking time. One of those scenes that shows just how much Dean loves his little brother, how lost he is without him and how he can't fathom the thought of living in a world that hasn't got Sam in it. Those feelings always lead to drastic actions that manage to make their lives even more fucked up than what they already are, but that's their eternal circle; self-sacrifising oneself for the other.