In the short story "The Secret Life of Walter Mitty" by James Thurber, the main character does not "make his daydreams become a reality," but rather his reality is altered by his mental wanderings. For example, each of his daydreams is triggered by something that he sees in his daily activities. I will list each of these below.
- His first daydream of speeding through a storm in a ship is brought on by his wife complaining that he is driving too fast.
- The second daydream of him performing surgery happens after he drives by a hospital.
- The next vision, the courtroom scene, stems from him seeing the "Waterbury trial" headline on the newsboy's paper; likewise, the last word of that sequence is "cur," a breed of dog, which reminds him of his wife's request that he buy "puppy biscuits."
- The second to last daydream where he is flying a war plane comes after he looks through war photos in a Liberty magazine.
- Lastly, and perhaps most subtly, the story ends with Mitty "[facing] the firing squad," which happens only after he is being pelted by raindrops.
So again, Mitty experiences these things only through daydreams; however, you could easily argue that since they are such vivid, almost debilitating visions, they do definitely alter his reality and life. After all, several of the dreams ended only because something potentially bad was happening: he was speeding, he failed to accelerate after a red light turned green, he went in the wrong parking lane, and his wife could not find him where he had sat down to wait for her.
If your question was more aiming at the cause of these daydreams, that is a trickier question. At the beginning of the story, his wife says, "It's one of your days. I wish you'd let Dr. Renshaw look you over." This means that she believes (or perhaps knows—we as readers cannot be sure) that something medical is going on with Mitty. Plausible answers could include PTSD, schizophrenia or general mental illness, brain tumors, etc., or maybe she and the above answers are all wrong and Mitty is just a vivid daydreamer! We really have little to go on with this other than the wife's quote, which is still a flawed theory since we are not given any accompanying information.