Big changes occur because all stories are ultimately about change. Somebody wants something, tries to get it, and something changes–that’s the essence of storytelling. Beyond that, what you’re asking about is probably too broad a question to answer–the reasons for doing MARVEL NOW! (we had a number of writers finishing up long runs on titles, so it made sense to do a round-robin creative switch and promote it) and with changing Spider-Man (Dan Slott had an idea for a story about Doctor Octopus killing peter Parker and taking over his life) are completely different, apart from teh fact that they’re all designed to entertain you and to part you from your hard-earned money (or shekels, as Stan would’ve said.)