This is because he's not quite smart enough to realize that wanting a Mercedes at a previous point to being subjected to misery because he's still to this day convinced that his idiot brother in law is some sort of expert woodsman means absolutely nothing. There is no more logical connection to the two events than there is to Chewbacca's presence on the moon of Endor to the Deflategate scandal. Since he is a stupid clod who believes in sympathetic magic, his belief that people need to have the perspective he only thinks he gained leads us to his renewed love of his favourite "Shut up, THAT'S why" argument: the appeal to shame.
We can thus connect his hateful comment about problem hair because he thinks that he's somehow magically owed children who smile and never worry about anything and his belief that Mike is too entitled and thus has to work on Exile Farm to learn what the real world is like to his mistaken belief that he's achieved some sort of wisdom. After all, he still does want that Mercedes. They can't have their Mercedeses but he's allowed his because he deserves more. Heh. I preferred Don Draper's epiphany that he was an ad-man through and through to John's wisdom.