Adam Alter in The New Yorker's Currency blog:

According to a great deal of research, positive fantasies may lessen your chances of succeeding. In one experiment, the social psychologists Gabriele Oettingen and Doris Mayer asked eighty-three German students to rate the extent to which they “experienced positive thoughts, images, or fantasies on the subject of transition into work life, graduating from university, looking for and finding a job.” Two years later, they approached the same students and asked about their post-college job experiences. Those who harbored positive fantasies put in fewer job applications, received fewer job offers, and ultimately earned lower salaries. The same was true in other contexts, too. Students who fantasized were less likely to ask their romantic crushes on a date and more likely to struggle academically. Hip-surgery patients also recovered more slowly when they dwelled on positive fantasies of walking without pain.