It wasn't easy for Bill Thielker to believe it when his doctor diagnosed him as depressed--mostly because he wasn't terribly sad. The 54-year-old landscape photographer and graphic designer felt lousy, all right--empty, unmotivated, detached from the people around him. But that was more or less how he'd always felt. "It was normal for me," he says. "I didn't realize anything was wrong. I just assumed life sucks and that's that."
If Thielker was depressed and didn't know it, he was not alone. More than 14 million adults in the U.S. have suffered a major depressive episode in the past year, and...