This is an excellent and very interesting question. As you know, Alonso Quixano (the main character's birth name before changing it himself to Don Quixote de La Mancha) was a retired man, nearing his 50's, who lived in the country accompanied by a servant and a niece. If we look closely at his character you can tell that this man was feeling the beginnings of a depressive state perhaps caused by agricultural depression (isolation from the city, as he lived apart in the country), and probably as a result of loneliness.
From seeing his former life prior to his "fit" of insanity, we can almost conclude that his transformation was necessary for him to occur. Other than that he would have remained in obscurity wondering what was missing from his life. He may have picked up a menial task but, as a man with some means, he would have become disdainfully bored.
He was obviously a man with a heart for adventure and it would have been criminal nearly to deny him the opportunity to chase his dreams.
Don Quixote was a gentleman with a decent estate, which would have been better kept if he had not spent his money on Romantic books of chivalry. His thirst for Romance would have to be satiated in some way or another, whether it was endagering his life or others as a knight or not.
In Ch. 6, the licentiate (academic) is searching through the titles of his library before they burn them, keeping a few valuables. When books of poetry are left, he suggest keeping those there, for they are not in the harmful addictive topic of chivalry. The niece and he then decide that it best not leave those either, for he may decide, if not knighthood, then to observe pastoral life as a poet, which may be worse.
So, DQ would find his outlet somewhere to fulfill his passionate, Romantic soul.