I believe that there are two possible answers to this question. One answer focuses on the more immediate question of what causes or triggers each imaginary fantasy, and the other answer looks toward the deeper seated root cause of why Mitty is having so many daydreams in the first place.
Mitty's daydreams aren't random. They are triggered by something in the environment around him. When he sees a hospital, he imagines that he is an expert surgeon. When he hears about a court trial, he is a fantastic lawyer. When he sees pictures of bombers, Mitty becomes a brave pilot. When he lights up a cigarette, he imagines himself standing before a firing squad.
The fact that Mitty is having daydreams shouldn't surprise or shock readers. We've all had daydreams, and more than likely, they involve us doing something in and around the environment that we were present in at that moment. What readers should notice is just how often Mitty has his daydreams. They are constant. It's possible that Mitty has...
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