As Christopher Robin begins to fill his head with more and more information, he has to physically leave the forest. (This is a theme as old as time. Remember the first story about people who to had to leave the perfect, beautiful garden once they knew too much?) Now, one could argue that knowing how to build a pump (House.10) is not necessarily worth the sacrifice of childhood imagination. On the other hand, children do grow out of the stage of fantasy play eventually. And Milne even calls what CR does in this setting "nothing."
Of course, nothing is a very good thing in Milne's eyes. Just read the book—wait, you have read the book—and you'll see how many wonderful somethings emerge from all that nothing. But in the end, children grow more interested in real information. In fact, theorists like Jean Piaget tell us that doing "nothing" is really how children start to learn about all the real somethings that eventually eclipse the nothing. But clearly Milne views growing up, inevitable and wonderful as it is, as a loss of something precious, as well.