But, they're filled with a dedication to law and goodness. Which, within the context of the story, so was Don Quixote. While he certainly behaved in ways that would seem more chaotic from an outside viewpoint, he was a knight on a holy quest.
That kind of dedication to a cause is a little crazy by itself.
That Quixote was towards the end of life and seemed to be in an altered state of mind, his inner self was pure, which makes him a paladin, sworn to an oath.
Moreover, his quest was successful. He achieved what he set out to do.