Why does Voltaire attack optimism in Candide?

Question

Is it just to prove Leibniz wrong or is he trying to change something, like the church or the way people are being treated?

Answer 1

He's attacking a very particular position, which says 'We live in the best of all possible worlds' - this came as an answer to the ever-nagging question: "If God is all-powerful and benevolent, how com there is still evil in the world?" So one answer was that we all lived in the best world possible. This world may not look perfect, but it is perfect insofar as there is no better possible world.
Voltaire opposed this view as part of his beliefs in humanism; the idea that man was mainly responsible for his condition (and not God) and that man would have to work to make his world better. So Candide is an attack on a kind of optimism that says everything that happens is for the best and we need not do anything to alter our condition.

Answer 2

At the time Voltaire wrote, the irrational optimism of Gottfried Leibniz was enjoying a wild popularity. This was in contrast to the religious intolerance, widespread disease, and warfare that characterized life in the 18th Century, and Voltaire felt this had a counter-productive effect on the spirits of Europeans of his time.

Answer 3

It is this hopeless optimism, that moves ahead, despite what is happening. "The Best of all possible worlds"