What does Voltaire see as the limitations and dangers of optimism?

With the theme of "Candide" as the human condition, Votaire sees optimism as a stutifying limitation upon understanding this condition.  For, Dr. Pangloss's doctrine of optimism holds that everything in life has reasons for its existence in the "best of all possible worlds."  Instead, through the misadventures of Candide, the world is not, the best, but absurd.

Voltaire does not simply protray Dr. Pangloss's thinking as foolish; he perceives it as dangerous because such thinking allows people to justify any inhumanity since "there is a reason" for it.  This ignoring of inhumanities prevents people from alleviating the suffering of others or justifies them.  Voltaire demonstrates the ridiculousness of this thinking in Chapter IV when Candide meets his former philosophy teacher, who is impoverished.  Thinking he is giving alms to a beggar, Candide is instead addressed by Pangloss, who describes his horrible conditions, telling him that Cunegonde (Voltaire's love interest) is dead, having died by being

disemboweled by Bulgar soldiers after having been raped as much as a woman can be.  They smashed the baron's head...the baroness was hacked to pieces...not one stone was left standing  [in the castle]

After this horrific description of what has happened, Pangloss states,

But we were well avenged, because the Avars did the same thing to a nearby estate that belonged to a Bulgar lord.

Voltaire further demonstrates the deadly effects of this thinking when Dr. Pangloss prevents Candide from saving Jacques the Anabaptist, who falls overboard during a storm, thus

proving that the bay of Lisbon has been formed to expressly for this Anabaptist to drown in.

This bold absurdity excuses Dr. Pangloss's passivity and lack of courage to risk death in order to save another human being.  Pangloss's rationalizes explanations for his behavior repeatedly.   Critic Darren Felty writes in his "Candide:  Voltaire's Satirization of Optimism and Pessimism,"

By presenting many such moments, Voltaire makes the philosophical justification of rampant injustice and destruction into a caustic joke of seemingly cosmic proportions.