Unit 3.A term that caught my attention

A term that caught my attention in this unit was "POLLYANNA Syndrome". It means : "a person regarded as being foolishly or blindly optimistic" after the heroine of the novel "POLLYANNA" by Eleanor Hodgman Porter (1868-1920), american writer. (thefreedictionary.com).
Is difficult for me to assimilate that extremely optimism could be as bad as pessimism. even though,we have to be realistic,what's wrong with to be Pollyanna? I don't understand!
Well, maybe some people annoy about this type of behavior or personality. It is the only negative thing that I  perceive.