Umm, it's pretty logical. It also means they want your wonders and their effects, which is a very reasonable reason to attack or - at the very least - dislike someone. I mean, I feel the same way as AI - if you build Chichen Itza and I'm Persia/Brazil, you're not going to be my friend any time soon. If you're nearby, I may plan to... take the city with it EVEN if I initially planned to be peaceful.