The cruelty in John’s choice to saddle Dean with this responsibility is staggering. And how incredibly selfish it was. After watching him in the last few episodes of season one, especially 1.22, hearing how ready he was to die, how tired he was of everything, his choice to trade his life for Dean’s can only be seen as a selfish one. He knew there would be a difficult choice to make regarding Sam because of the demon blood, and when he realized he could make a deal for Dean’s life, he saw a way out, so he took it. He didn’t want to make the choice to kill his youngest son, so he gave that task to his eldest instead. One final reason, in a long line, for Dean to believe his life is worth less than Sam’s.