This story from NPR is disturbing (to me) but not exactly surprising: eating a lot of chocolate is linked to depression.

So three researchers from the University of California, San Diego asked 931 people who'd come in for an unrelated study about cholesterol how many times a week they ate chocolate. The people also filled out a depression questionnaire.
People in the group with screening scores suggesting that they might have major depression ate 12 or more servings a month.

Why?

It could be that people are self-medicating. The researchers can't say that on the basis of this study. Other possibilities: depression may somehow initiate chocolate cravings, or chocolate may trigger depression (though they note this isn't likely).

My pet theory: women are more prone to depression, and they are more likely than men to go for chocolate versus the salty stuff. But apparently using chocolate as an anti-depressant doesn't work, at least not in the long term.