Why bad news dominates the headlines

When you read the news, sometimes it can feel like the only things reported are terrible, depressing events. Why does the media concentrate on the bad things in life, rather than the good?
 And what might this depressing slant say about us, the audience?
 It isn't that these are the only things that happen. Perhaps journalists are drawn to reporting bad news because sudden disaster is more compelling than slow improvements. Or it could be that newsgatherers believe that cynical reports of corrupt politicians or unfortunate events make for simpler stories.
But another strong possibility is that we, the readers or viewers, have trained journalists to focus on these things. Many people often say that they would prefer good news: but is that actually true?