The horrible strip in which John plays the filthy problem hair stunt on Lizardbreath and the equally horrible strip in which he declares that Lizzie is too young to be sensitive about her looks and the especially horrible strip in which he tries really hard to not understand why Liz feels so lost and unhappy remind us that John wants very much to live in a sort of bubble of not having to deal with things. I'm reminded of lyrics from a really, really old song by Chicago back when they were pissing people off by adding the words 'Transit Authority' to their name: "If you had my outlook, your feelings would be numb, you'd always think that everything was fine...EVERYTHING IS FINE!!!" Simply put, his detachment, his serenity, his life of idiot glee is motivated by his very real need to blind himself to the problems around him because he's too squishy inside to deal with life.
This means that marrying someone who wears her heart on her sleeve like Elly is going to be a rather troubling proposition. There's no one who feels more and with more sincerity than Elly does and her non-stop need to face issues that scare the shit out of him because all roads lead to him getting what he wants being the grand unifying theory of why his life sucks and why his kids act the way they do. Acknowledging a problem means acknowledging the possibility that the problem is him so he'd rather not be subjected to the humiliation of being always wrong and also a bad person.