My own idea was shaped by the fact that I had just been running a Changeling game, and had realized during the course of it that I did something far crueler than I meant to in the story. By Changeling mechanics, humans can only see chimerical (faerie) things if they're enchanted, and after the enchantment wears off, they forget a great deal of what they saw and did. I needed the human who was helping the player-characters to actually be useful, so I gave him a gift that allowed him to remember it all clearly -- but didn't think until much later what it meant that they kept enchanting him, and that it kept wearing off. He essentially was dropped repeatedly into Oz, where everything went full technicolor, and then got booted back into black-and-white Kansas again, to pine for what he had lost and wonder if he was crazy for thinking he'd seen it.

And that was what was in my head when I re-read the Narnia books before seeing the PC movie. No wonder my brain went straight to interpreting Susan's adult behavior as a defense mechanism, after Narnia abandoned her the second time.