Nobody likes a Debby Downer, and supervisors are no exception to that rule. If you were to ask every manager on the planet what the idea employee looks like, they would likely give you a similar picture. The employee would be hardworking, cooperative, helpful, a problem solver and would not be a complainer. If you constantly whine about having too much work, being too tired, hating Mondays, wishing it was Friday, loathing that the computer is running slow, getting stuck behind terrible drivers in the morning or any other inane thing, your manager is going to want to stick a piece of duct tape over your mouth simply to make the endless barrage of negativity stop. Your coworkers are likely to want to do the same. Not only is constant complaining unprofessional, it can drag everyone else down, too. So, your minor case of the Monday blues suddenly has the entire office hating life and, as a result, getting nothing done. That is not the sort of person any manager wants on their team.