John Winchester, is pretty much like Dumbledore in this fandom. His curse was that he understood too much. he was too wise, too broken and too strong.
Everyone keep saying that John was a bad father because he introduced his sons into this life. but guess what?! he didn't! he didn't introduced them to this life, they were already a part of this life. They were meant to be.
As that Cupid said, It was a big deal getting Mary and John to fall in pag-ibig so that Sam and Dean are born to be Lucifer and Micheal's vessels to fulfill destiny of this world, the apocalypse, God's plan.