The main theme running throughout the novel is good versus evil. Harry Potter represents the good, whereas Voldemort is evil personified.
Harry's upbringing as the oppressed foster child raised by relatives in a very conventionalized English household forms him into an unspoiled and compassionate adolescent who knows what it is to suffer and be treated unfairly. He is very grateful for the friendship, love, and community he enters into in Hogwarts—the simple nurture and caring that he receives in the wizarding world is new to him. He is, as a result, a well-balanced individual who does not have an inflated ego or high self-opinion despite his importance in his new world.
He stands for good because of his attitude of basic goodness, decency, kindness, and gratitude to others. He attracts to him like-minded people who are unpretentious and caring.
Voldemort, filled with anger and bile, is the opposite of everything Harry represents. He and his followers want to oppress others, rob them of their souls, and make them suffer. If Harry is about love and humaneness, Voldemort is about sucking all the joy out of others and dominating them completely. He revels in sadism and cruelty.
In this epic battle between good and evil, the soul of the wizarding world is at stake. If Harry's side doesn't win, everything good about the wizarding world that has been built up over the centuries will be destroyed. The stakes are very high, and Harry seems like an unlikely leader for this battle.