What causes Walter Mitty's fantasies?

It is interesting to identify just the things that evoke Walter Mitty's fantasies throughout the story. One which can be easily overlooked is especially subtle and seems intended to tie the story together with a beginning and an ending. In the opening paragraph Commander Mitty, in full-dress uniform, is not driving an automobile to Waterbury on a shopping trip but piloting a "huge, hurtling eight-engined Navy hydroplane" through stormy seas, shouting such absurdities as "Switch on No. 8 auxiliary!" and "Full strength in No. 3 turret!" An officer warns him, "We can't make it, sir. It's spoiling for a hurricane, if you ask me." "I'm not asking you, Lieutenant Berg," said the Commander. "Throw on the power lights! Rev her up to 8,500! We're going through!" No such boat ever existed, of course, and a hydroplane with eight engines sounds like an utter impossibility. But what is most interesting is the warning that "It's spoiling for a hurricane."
Walter Mitty must have noticed a few raindrops on his windshield, giving warning that it was going to rain before they got their errands run and returned home. At the very end of the story, while Mitty is waiting for his wife outside the drugstore, "Walter Mitty lighted a cigarette. It began to rain, rain with sleet in it. He stood up against the wall of the drugstore, smoking." This is an admirably artistic way to begin and end the story. The approaching rainstorm and the rain and sleet finally falling seem to round out what could otherwise be an open-ended tale with many other untold fantasies preceding and following those that are dramatized. The end is vaguely reminiscent of James Joyce's story "The Dead," which ends with this beautiful passage:
A few light taps upon the pane made him turn to the window. It had begun to snow again. He watched sleepily the flakes, silver and dark, falling obliquely against the lamplight. The time had come for him to set out on his journey westward. Yes, the newspapers were right: snow was general all over Ireland. It was falling on every part of the dark central plain, on the treeless hills, falling softly upon the Bog of Allen and, farther westward, softly falling into the dark mutinous Shannon waves. It was falling, too, upon every part of the lonely churchyard on the hill where Michael Furey lay buried. It lay thickly drifted on the crooked crosses and headstones, on the spears of the little gate, on the barren thorns. His soul swooned slowly as he heard the snow falling faintly through the universe and faintly falling, like the descent of their last end, upon all the living and the dead.