My take on it is that Jack's ability to remain sane is linked to his ability to remain alive (or to not remain dead), and it's tied to Jack's being a Christ-like figure in the narrative.

Jack's something more than human, and just as his body doesn't retain the violence or damage that should kill a normal human body, his mind doesn't retain the damage that would come from being underground for so long. He's just wiped clean somehow--so that he's always pure, always whole, always capable of absorbing the wounds of other people and absolving them of their pain.

He couldn't be a savior without that.