Why Don Quixote becomes a knight-errant in Chapter 1?

In Chapter One, we learn that Quixana is a man who has of late become so enraptured of his leisure activities that he is leaving all his responsibilities fall by the wayside. His chief passion in leisure is to ingest all of the tales and books and poems involving knights, chivalry and adventure that he can possibly lay his hands on. He spends time discussing these books with his friends from the village, the curate and the barber. So all-consuming becomes his pastime, and so poorly and illogically written are the books he is poring over, that eventually Quixana loses his head, and he suddenly decides he is, himself, a knight and adventurer in the same vein as those he has been reading about in his books.

After Quixana designates himself as a knight, he begins fashioning himself some armor and a helmet from some antiques and materials he found around his house. He christens his old, beaten-down horse Rozinante, fitting of a lofty steed. He chooses to call himself "Don Quixote of La Mancha," following the manner of the characters of his stories. He finally chooses the lady for whom he will be embarking on his quest, a country girl named Aldonza Lorenzo whom he renames Dulcinea of El Toboso.

The main metamorphosis takes place in Chapter One, as the protagonist convinces himself that he is Don Quixote, knight-errant extraordinaire, questing on behalf of his love Dulcinea. The author also introduces the mindset that will allow him to be Don Quixote in a world where such a man doesn't exist. He believes in nothing but the truth, even if the truth he sees isn't the truth in reality.