One cannot simply be told how to improve their behavior because it takes away their choice in the matter. It feels forced. As soon as you’re told to do something differently there’s an initial reaction to hold resentment towards whoever told you.
Your parents can’t just tell you what the “right thing to do” is. In order for you to believe them, you must consistently be shown through their actions.
Imagine your father explaining to you that being honest is the best way to live a happy life. Then, you observe him telling the truth in a tough situation with your mother. He admitted to giving you a joint to smoke at a party, and you know she blatantly told him not to encourage pot smoking. It would have been much easier to tell a “white lie” but he values honesty and told the truth. No matter what argument comes out of this, your parents’ relationship with each other, and with you will likely be stronger because of the honesty and teaching through actions.