"Fantasy is escapist, and that is its glory. If a soldier is imprisoned by the enemy, don't we consider it his duty to escape?"
This quote is recorded in several writings of Tolkien including "The Tolkien Reader". I find it sums up Tolkien's motivation behind all his sub-creation of Middle Earth rather well. When people would ask, or even criticize him, about the lengthy hours and obsessive moments spent over his many works, this is how I would imagine him responding.