Schopenhauer's pessimism comes from his elevating of Will above reason as the mainspring of human thought and behaviour. Schopenhauer pointed to motivators such as hunger, sexuality, the need to care for children, and the need for shelter and personal security as the real sources of human motivation. Reason, compared to these factors, is mere window-dressing for human thoughts; it is the clothes our naked hungers put on when they go out in public. Schopenhauer sees reason as weak and insignificant compared to Will; in one metaphor, Schopenhauer compares reason to a lame man who can see, who rides on the shoulders of the blind giant of Will.
Likening human life to the life of other animals, he saw the reproductive cycle as indeed a cyclical process that continues pointlessly and indefinitely, unless the chain is broken by too limited resources to make continued life possible, in which case it is terminated by extinction. The prognosis of either pointlessly continuing the cycle of life or facing extinction is one major leg of Schopenhauer's pessimism.
Schopenhauer moreover considers the desires of the will to entail suffering: because they are desires; because their objects are always limited resources; because other living things must be excluded from those resources. The business of biological life is a war of all against all. Reason makes us suffer all the more, in that reason makes us realize that biology's agenda is something we would not have chosen if we had a choice, but is helpless to prevent us from serving it or failing to feel the sting of its goads.
The term has also been used to describe the position of the Norwegian philosopher Peter Wessel Zapffe, although he clearly states in his philosophical treatise Om det tragiske that pessimism is a term which cannot describe his biosophy (partially inspired by Arthur Schopenhauer).