These people have sand in the gears of life.  The pessimistic bias in their thinking colors their world.  Their entrenched pessimistic outlook makes life more difficult than it really is, and certainly harder than it needs to be.  Pessimism isn't all bad.  At least they aren't clinically depressed.  Although they may look on the dark side, they don't have significant problems with command and control, and twisted thoughts and distorted emotions don't rule their mind.

It's hard to turn a pessimist into an optimist because they need a mind transplant.  they need total reprogramming of their mind with a positive operating system.  Pessimism works for them, and they really don't want to change.  Although they may not enjoy their life as much as an optimist, their comfort zone easily accommodates their pessimistic world view.

Pessimists don't need to be on medication or to talk to a therapist.  As long as they  maintain command and control, and they don't have a problem with twisted thoughts and distorted emotions, they can live in their pessimistic world.  but if their pessimistic outlook is wearing thin, and they would like to have a more positive point of view, change is possible.  The Bible says, "Be transformed by the renewing of your mind."  The wheel of change turns in the direction of what they put into their mind, and if they start filling it with positive things, they can push their life in a positive direction.  Their pessimistic world view can dissolve, and they can actually come to believe there is no limit to how good their life can become.

Excerpt from :

Zero Tolerance To Negative Thinking : Good-by Depression - Hello Positive Mind

David J Abbott M.D.