In the time when the novel is set, it was not unusual for noble-born peoples to place gods in their bloodlines. Greek culture had a long tradition of gods breeding with men to produce heroes: Zeus, the father of the gods, had dozens of children with mortal women.
Alexander's mother, Olympias, believes him to be the son of Zeus instead of his father Phillip. Because of her beliefs, Alexander is raised to believe himself special and unique, a man destined for greatness, and so he learns and prepares himself for his eventual rulership all his life. His mother encourages him, while his father insists he focus on real things; because of the love he has for both parents, Alexander grows distant from them since he cannot please both at once.
Alexander becomes a leader of men early, first as a surrogate for his father, then of his armies after Phillip's death. His natural inclination for leadership brings him power over men and the approval of Olympias, who always reminds him that he is god-born. He saw in himself no limits because of his heritage, and with no limits he saw no reason to stop finding new lands to conquer. All his motivation stemmed directly from Olympias's belief in the Greek gods.