Kaede can't process this because in her scenario the childhood friend never wins. She must have encountered that statement multiple times, considering she reads a lot, inlucing manga, and is obsessed with stereotypical childhood friendship. So, she would have been prepared to get really close to Kouhei, but then some day Kouhei would get together with some other girl, leaving Kaede behind. That's the trope scenario, which means it's also what Kaede must have had in mind. It's just a simulated childhood friendship she has with Kouhei, so it should follow a clear, planned path. We have seen her fulfilling objectives from a checklist before, just like with the latest "childhood friends must randomly meet during a school trip, when everyone else is already asleep."
