It's easy, really. All good things come in threes. These are the hewers of wood, and drawers of water. Justice does not emerge spontaneously. Certain characteristics re assigned to each of these three classes.
Justice demands certain cultivation of each nature.
Of course a raw propensity for wisdom or courage does not guarantee the emergence of wisdom and courage. While these two virtues, wisdom and courage are seated respectively in the ruling and auxiliary classes, Plato recognized a third virtue, moderation which runs across all three classes. They take care of and mind their own business. Injustice is the meddling or interference of the parts. I've taken a few liberties with Glaucon's one liners.