“You’ve got to accentuate the positive,” according to the lyrics from an old song, “and eliminate the negative.” While there are dangers in being a “cockeyed optimist”—we find those words in another old tune, this one by Rodgers and Hammerstein—hanging around a habitual pessimist can drain our spirits, dim our enthusiasm for a job, and wound our pride. Most of us know these sorts of folks, the men, women, and even teenagers who manage to find a black cloud on the sunniest of days. Mention that you’re hoping for a raise in pay in a couple of months, and they’ll just guffaw. Let slip that you’ve fallen deeply in love and hope to be married, and you’ll get a mini-lecture on the pitfalls of matrimony, with statistics on divorce thrown in for good measure. Declare that you’re off to the Outer Banks for a vacation, and your listener will remind …