He has become obsessed with books of chivalry, and believes their every word to be true, despite the fact that many of the events in them are clearly impossible.
Quixano eventually appears to other people to have lost his mind from little sleep and food and because of so much reading. Wikipedia
Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be!
Fortune leaves always some door open to come at a remedy.
For if he like a madman lived,
At least he like a wise one died.