There's lots of evidence about how positivity boosts productivity. Jane Dutton, David Cooperrider, Bob Quinn, and Amy Wirzesniewski come to mind. But it's easier to be negative, and may make others view you as smarter to boot!
Naysayers tend to deflate motivation and bring productivity to a grinding halt -- just to salve their needy egos. "Negative evaluation is a tactic people use when they're intellectually insecure," says Teresa Amabile, a professor of business administration at Harvard Business School. More than 20 years ago, Prof. Amabile conducted a study in which she discovered that criticism sounds smarter than praise -- that people believe lashing critics are smarter than the approvers.
When people evaluated edited excerpts from negative and positive book reviews, she found that negative reviewers also were seen as more expert and competent "even when the content of the positive review was independently judged as being of higher quality and greater forcefulness."