Commentators unanimously agree that Jesus uses this well-known proverb to reinforce His example of washing the disciples' feet. The logic is simple and powerful: if your Lord and Master humbled Himself to serve, you, as His servant, have no basis for considering any act of service beneath you. As Charles Spurgeon notes, if we feel too important for humble tasks, it's a sign we need more humility, which is the true path to grace.