Panel 1: When Liz asks John when the new baby will get there, he says 'some time in March' because Lynn had yet to be entranced by the phrase "April fools all of us."
Panel 2: When she asks if he's happy she's getting a new baby, John hints at the pay-off line when he says that he sure is.
This is because a human being who is aware of his surroundings and the people who occupy them would realize that Liz is a throbbing bundle of anxieties about her place in the world.
Panel 3: Once again, John flunks the 'being a human being', 'knowing who he's talking to' and 'awareness of what's going on around him and why' tests when he doesn't get the import of Liz pointing out that new babies are very small and replies that they need lots of attention.
Panel 4: This is why her heartfelt question of "Daddy? When the new baby comes, will you still love me?" comes as a baffling shock.
Summary: History teaches us that John is hampered by the limiting belief that every day is kid's day. He is not willing to allow himself to consider the possibility that children feel fear, anxiety or sadness because envy and stupidity make him think that he was the only child who ever felt bad about anything.