In Voltaire's opinion, man becomes corrupt because his own "happiness" is paramount to all else. He is a victim of his own appetities and too blind to see that often what he thinks will lead to happiness is actually unobtainable.
Some of the examples of illusory happiness of the human condition include war, rape, bigotry, and supersititon. In war, men believe that conquering others will lead to greater power which will make rulers, nations, and people happy. Rape is another form of power, obviously sexual, again with the false belief that this kind of "conquest" will lead to some sort of satisfaction. Bigotry is the result of feeling superior, and that if others weren't in the way, we'd be happy. Superstition is "magical thinking," belief that appeals to someone else in "control" will make humans ultimately happy.
Human beings seem designed to wonder what else is "better" out there that could ultimately make them "happier." As Candide says in Chapter 6, ""If this is the best of all possible worlds, what are the others?"