I've always wondered if that is the case. And if so, doesn't that mean there is a direct correlation between ease of living and depression? The less you have to worry about and deal with, the more likely you are to be depressed?
Humans were created to work, the harder they work the better they feel. Animals are the same way - if you give your pet an easy life, they will die early. Make them work (for example put their food where they can reach it, but have to work hard to get to it) and they live longer.