All In The Name

Because let me remind you, John knew. He knew what Sam was. He knew what Azazel’s plan for him. We found out in My Time of Dying when he’s bargaining with Azazel. 

We find out he knew Sam is potentially dangerous in Hunted, which means he probably knew Sam had demon blood in him. 

This to me indicates that at some point, probably long before the series started, John found out that the demon that killed his wife is also going to make his son into a pawn for the demon side in the apocalypse. This, to me, explains everything. This is why he never gave up. This is why at times the boys thought he put finding the demon over them. Not because he was mad with revenge, but because he couldn’t lose his son the way he lost the love of his life.