Diligence is a character-trait which is extremely necessary for learning the wisdom of the Torah and also to be able to enter into the fear of Hashem and the love of Him and to conduct oneself with good correct character-traits, as all of these things need diligence to learn the books of ethics which teach a person sense and to listen to studies day after day and night after night until the light of the Torah will bring him back to acting in the correct way. In the same way as a continuous drip of rainwater can pierce a stone, so too words of Mussar pierce the heart.
However, if he only studies works of ethics and Mussar or listens to their explanations once in a blue moon he will feel nothing and it will have no effect on him. Even if there will be a temporary effect it will immediately disappear and will have no long-term effect. The more that he increases his in-depth studies of Mussar and Fear of Hashem and listens to their explanations so too his wisdom, fear of Hashem, cleaving to Hashem and the perfection of his own character-traits will increase.