Candide is unfailingly optimistic after every disaster that befalls him. He always thinks that "everything is for the best" and that he lives in "the best of all possible worlds." He has been taught by his teacher, Pangloss, to believe that everything is for the best, so even after he is exiled from the castle where his beloved, Cunegonde, lives and is robbed and forced to fight a war and beg for food, he says in Chapter 3:
"It was necessary that I should be banished from the presence of Miss Cunegonde; that I should afterwards run the gauntlet; and it is necessary I should beg my bread, till I am able to get it. All this could not have been otherwise.”
Even after Candide and Pangloss witness the deadly Lisbon earthquake (a real event that happened in 1755) in Chapter 5, Pangloss still says that all has happened for the best, and Candide is in agreement. Voltaire is satirizing a school of philosophy called Leibnizian optimism through the characters of Candide and his teacher, Pangloss. Candide is clearly a subjective narrator who is not to be trusted as reliable.
Candide is young and naive, making him the definition of the unreliable narrator. The reader cannot trust the unreliable narrator to convey correct information because the narrator himself does not fully understand what is happening. Candide is overly optimistic, making trusting his judgment a problem. People take advantage of him because he is hapless and foolish, and though he is fundamentally a good person he seems to have no control over what happens to him. Chaos follows him, and he cannot be considered a reliable narrator because he is still trying to figure out life, and what is happening to him.