From Steve Myers' Poynter story:
Fuller explained that we're drawn to scary images like fires and car wrecks for the same reason our ancestors kept an eye on predators: survival. When we see such images, fear courses through our brains and focuses our attention on what appears to threaten us. Our brains respond the same way that our ancestors' did, even though we know we're not really endangered by those dramatic images.
An interesting discussion ensues over the extent to which journalists must use emotion to get readers' attention, and the ethical limits of this approach. It's worth reading, and so likely is Fuller's book, which comes out in May.