Mass extinctions have taken place at various times throughout the history of life on Earth, and will probably continue to happen. They seem to be an important event in the process of evolution, as they allow new life forms to spring up where voids has been created by the extinction. Think of it this way: if the Cretaceous extinction hadn't taken place, we probably wouldn't be here. And because we're here, like any other life form, we're going to be vulnerable to a mass extinction.