What caused Don Quixote to change in Chapter 1

There is an older gentleman (named Quixana or perhaps Quesada) and he lives in a Spanish village called La Mancha. As the story begins, this man has lost his wits. "His imagination was full of all that he read in his books"‹stories of medieval knights, chivalry, and bloody battles. As a result, he changes his name to Don Quixote and decides to become a knight-errant.