(Strip Number 5138, Original Publication Date, 26 May 1989)
Panel 1: It's John's turn to be a hypocrite who forgets the past when he tells Elly that if Mike knows that he's getting her angry, she'll lose control of the situation. Given his own over-reaction to "Did I ask to be born?" and stubborn refusal to admit that Mike isn't freeloading off of him because he wants more out of life than to rake leaves, this is purest twaddle from someone who speaks it fluently.
Panel 2: Elly shows John how to revise history like an expert when she tells him a nonsense story about how easy it was when they were younger. Back then, No meant no (despite what snarker-trolls might say about how she never backed up most of her threats of mayhem) but now, there has to be a reason for everything. Every stupid, over-the-top, inconsistent and arbitrary punishment for no other reason than "Elly is angry" has to become the subject of a debate on her competence and she wants to know why he pushes her buttons (but doesn't want the answer to be "Elly Patterson Is A Short-Tempered Dimwit").
Panel 3: John explains that all kids do that; it's a breaking away process that allows them to establish their independence before they leave.
Panel 4: Since Elly will always take "having a difference of opinion" to mean "trying to beat her to her knees and totally silence her", she glumly thought-bubbles that more of Mike telling her that she's being a jerk who plays favourites tempts her to leave first.
Summary: Since John doesn't pay attention to what's going on in his own house and also doesn't care to know, he probably makes the mistake of taking Elly at her word when she says that suddenly, Mike is a problem. He doesn't have the advantage of having read this strip and seeing that Elly never got along with Mike in the first place.