I think this is actually at the root of why Dean is so unforgiving toward Sam. He's had 'you are responsible for Sam' as his foundation for so long, that I think somewhere in there, he thinks Sam's mistakes are his own, and so forgiving them would constitute forgiving himself. Which he's not good at, to say the least.

Interesting theory! It's pretty clear Dean can't forgive himself, so it makes sense that if he considered Sam's mistakes his own responsibility, he couldn't forgive him either.

Ouch. :( Though in some ways, I think it's even sadder that young Sam who used to get angry at John so easily is now too weary to stay up and fight with Dean.

Yes. Too worn down, physically and emotionally, to fight about it any more.