This is a good topic, because this one girl absolutely denies me every time I just ask her out to lunch or something, not even anything romantic. I'm thinking that it's because she has a BF, but again, I hate getting dissed just asking to do innocent things with some of these girls. I just get along with her so well, and she's easy to talk to.
Perhaps some girls turn down stuff with you because they actually don't want to do anything that might land THEM in the LJBF zone. Then again, maybe not. Who knows, it's a mystery. Any experiences to share?