Walter Mitty is absent-minded. He lives in a world of daydreams. He is not in tune with the modern world in which he actually has to live. Like most absent-minded people he is incompetent to deal with reality and frequently makes mistakes. And like many absent-minded men he is dependent on a wife to take care of the practical necessities of their joint existence. She has to tell him what necktie to wear and not to put on his black shoes with a brown suit--that sort of thing. Mitty secretly hates her but knows he couldn't get along without her. Mitty's absent-mindedness shows itself conspicuously in one part of the story where he is trying to remember one of the things his wife told him to buy. Then when he manages to remember, he says the words out loud. They are "puppy biscuit."
Human males evolved to be hunters, explorers, warriors, and adventurers; but modern civilization formed by the Industrial Revolution has deprived most of them of the need to fill such roles. Instead they work in offices from eight in the morning until five at night and then come home to their wives bringing a paycheck instead of an animal carcass. Many men lose themselves in television and alcohol. They especially like to watch sports, because at least the men playing football and other rough games are behaving somewhat like real men. Their frustration explains the popularity of TV shows like Survivor and the infinite number of cop shows. Walter Mitty is superior to the average male couch potato because Mitty--like Thurber himself--can escape into his own imagination.
Mitty resembles Leopold Bloom in James Joyce's novel Ulysses. Throughout that book there is a comical contrast between Bloom, the modern man, and Ulysses, the intrepid hero of Homer's epic poem The Odyssey. Freud dealt with the alienation of modern man in his long essay Civilization and Its Discontents.
Using the concepts of the superego, the sense of guilt, and the aggressive instinct, Freud formulated the main theme of Civilization and Its Discontents: the ineradicable antagonism between the demands of the individual’s instincts and the restrictions of civilization.
Thurber’s friend and sometime collaborator E. B. White uses similar ideas in his stories “The Door” and “The Second Tree from the Corner.” White tried to escape the dehumanization and alienation of modern civilization by moving to a farm in Maine.
It seems significant that the Mittys do not have any children. They have a puppy instead. If Mrs. Mitty had a baby, she might stop babying her poor husband.