How can people not be depressed when they watch or read the news?

I was reading about the floods in Nashville, the Joplin tornado, and the earthquake in Japan and it all makes me so sad. Not to mention the murders and cruelty (to people and animals) that goes on. How can people hear about all these things and not get depressed? I've done all I can to help victims of the natural disasters (donated money and time), but I wish I could do more and save everyone.