Pessimism is a human mindset, which tends to only see the negative things in life. A pessimistic person will tend to expect and assume the worst, having a "glass half-empty" personality. Pessimism is very often the cause of depression.
However, a small amount of pessimism can be beneficial, such as avoiding scams with the "if it seems too good to be true, it probably is" adage.
- Learned Optimism by Martin E.P. Seligman, Ph.D.