optimism is widely acclaimed as a positive quality. everybody exhorts you to acquire a positive bend of mind, be it finding the best characteristics of a neighbor, the qualities of a product or the plus point of a work of art. certainly this is one thing that you need to succeed in life.
but pessimism also has a certain role in human life. critical approach to life, the events and persons you come across has to be developed if you need to avoid being taken for a ride. suppose you are on an interview board, to recruit some people for the organisation you are working for. of course you have to find the positives in the people you interview. but at the same time you have to see through boasters, tricksters and laggards. for that you have to pay attention to the negative side.
another example is when you read a piece of literature. there is no rule that you should read cover to cover any useless article you come across. a critical mind helps you to distinguish between good and bad. if you find it is not worth your time, you can discard the stuff at the earliest and save your valuable time. this applies to people too. if you can evaluate people within the shortest time , you avoid a lot of worrisome moments later. so critical thinking is a shade better than pure positive thinking in practical life.