Because, as we know, almost anything can be read into any book if you are determined enough. This will be especially impressed on anyone who has written fantastic fiction. He will find reviewers, both favourable and hostile, reading into his stories all manner of allegorical meanings which he never intended. (Some of the allegories thus imposed on my own books have been so ingenious and interesting that I often wish I had thought of them myself.) Apparently it is impossible for the wit of man to devise a narrative in which the wit of some other man cannot, and with some plausibility, find a hidden sense.
In defense of my own tendency to see deep truths of Christ and signs of God's sovereignty oozing out of almost every news story, book or movie, and personal experience, I pray that it might be a sign that I am beginning to "bleed Bible" when pricked. (Remember this?)
The Lewis quote comes from his Reflections on the Psalms, "Second Meanings."