I feel like waiting for the third book is part of the story.
We're supposed to wait and wonder and agonize over the meaning of it all and where it's going to go. You're making is grow while we think about the box what might be in it.
Maybe he became consumed by the puzzle and lost parts of himself a little more every time he let it become his main focus.
Maybe we all do that a bit. Let the big, scary, confusing puzzles that "keep us from moving forward" take control of us. Drive us from our friends and family. Our real goals. Happiness. Maybe we get stuck trying to solve an impossible riddle, when all we needed to do was set it aside and let our sleeping brains do the heavy lifting while we sing tone deaf karaoke with our friends. Eat some nachos. Prank call the white nationalists. Sets some bags of poop on fire.
All of his energy is spent trying to lift a stone that wasn't meant to move. He's exhausted and can't hold the toothbrush long enough to clean his teeth. He used to have a nice smile.