"Researchers have also found that people in a negative mood produce better quality and more persuasive arguments than people in a positive mood, and that negative moods can improve memory."
Have you ever been the "negative one"? Have you been coached about your negative attitude? On the other hand, have you had to coach someone who was being negative?
This article from Newsweek, "The 'Tyranny' of Positive Thinking can Threaten Your Health and Happiness", is a helpful reminder when a new idea takes hold and takes off when it gets simplified for the masses.
Positive Psychology has many facets and unfortunately has been simplified in the business world to "positive thinking". This pervasive argument that if you think positive you can overcome anything. The simplicity of the approach has quickly vanquished the fact that some people respond to a negative approach. There may be a need to look at the problems and issues as the approach instead of treating those that approach situations with a negative focus as wrong, disruptive or not being able to move a project forward.
So, before you dismiss that argument from the "negative person", you might want to have a better understanding of how they process and interpret a situation.