Panel 1: As he and Liz get ready to watch television, Mike is dismayed to find that Elly has taped a note to the screen telling him that he has to do his chores first. Since he remembers that they can never be done to Elly's satisfaction and that when she criticizes him for failing her because he hates her, she talks to him as if he's still the four year old who howled that nooooooobody loves him, this irritates him.
Panel 2: He expresses his irritation by asking what is their problem anyway. He believes that he does not deserve to be treated like this because he's the same size as Elly and mistakenly believes that he looks like an adult.
Panel 3: He then opens himself up to a smart remark by howling "Why does everybody treat me like a child?!!" because he won't admit that he still is one.
Panel 4: Liz's reply of "Maybe they're just waiting for your brains to catch up to your body" leaves the dope looking like a gaffed trout.
Summary: Here's a lot of Mike's problem right here: he thinks that he matters more than he actually does. He doesn't so much outgrow this sort of thinking as he does change how he expresses it. This allows him to wail idiotically about being a rung on a ladder because he won't admit that yeah, he has to pay his dues writing puff pieces like a boring, ordinary person.