I've been thinking and maybe John''s obsession wasn't about revenge for Mary as Sam thought. Mary was fine until she entered the nursery meaning she encoutnered whatever killed her because she entered the nursery where Sam was sleeping. So naturally John would of thought whoever killed Mary might originally been after Sam and she interruppted causing her death. Maybe his obsession wasn't revenging Mary's death but protecting his children. He knew he couldn't bring back Mary from the dead but maybe he could keep whatever wanted Sam from getting near again and possibly Dean. Maybe that's why he was so hard on Sam. He already admitted to wanting the boys to be ready just in case he couldn't be there to protect them so maybe he was trying to drill into his fear of whatever was out there that was after Sam and had killed Mary. It's likely the Shtringa caused all those fears to bubble back up since it does go after the youngest and work it's way up the food chain to the older siblings. He might of been afraid that it was Mary all over again. Not to mention Dean lived with the fact that in those brief moments of need his brother nearly died. It more then likely affected John the same way. A parent duty is to protect a child. As long as the child is protected he's alive and everything else can be done like teaching and so forth. If you aren't a parent you don't know until you are and if your a parent whose never lost a child be grateful for your blessing. A parents worst fear is losing a child and there is no pain like knowing there was nothing you could of done to save that child and yet you still blame yourself for it. Younstill punish yourself daily with the burden of your 'sin', tour failure to protect the gift that was given to you. Like Dean said in All Hell's about the one job he'd been given and yet he let Sam down. I could hear hthe echo of John in those words. What do you think?