Human suffering in Candide comes from all the usual places: war, greed, envy, lust, and pride. Evil stems from humans' desire to be more powerful than others. Whether it motivated by class, political or religious power, or reputation, humans conspire against each other to move a step higher on whatever ladder they seek to climb.
European society is contrasted with El Dorado, a utopian civilization where everyone has enough and the trappings of wealth and power in Europe are scorned as rubbish. Because everyone has enough and no one can gain power or status over anyone else, they are happy despite difference. When Candide leaves this utopia, he does so because he wants to have prestige in the debased Europe he has left.
The ending suggests that the principal characters do achieve a measure of happiness when they reunite in poverty and sickness to earn their living by eking out simple life through their labor in their garden. To "tend one's garden" is to employ one's energy and gifts in the tasks immediately at hand, to do one's best in the small sphere in which one is placed rather than to worry about larger problems, and to take pleasure in one's sufficiency rather than to construct grand narratives about the mystery of the world.