Perhaps Voltaire was not so much a pessimist as a disappointed idealist. Such is usually the case with satirists: They see what problems exist in their society and they ridicule them with the hope that a remedy of the ills of the society can be fixed.
The 1600's were a period of literary criticism. With the establishment of L'Academie Francaise, France had the first official "court" of usage and literary taste. Voltaire was a man of his times. He also was imprisoned and beaten on more than one occasion. His imprisonment was unjust, done in the name of preserving the prerogatives of the aristocracy. So, Voltaire's criticism was often to undermine the corrupt system.
For one thing, in Candide he writes that the group continue their search for the right place despite finding a Utopia in El Dorado. Voltaire portrays the human condition and ridicules man's insistence on believing in things despite the contradictions of Nature and "the system." In short, Voltaire satirizes the insitence that "all is for the best.," the maxim of some of his contemporary optimists.