He who loves purity of heart,
and whose speech is gracious,
will have the king as his friend.
Note the addition of gracious speech. Shouldn't purity of heart be enough for friendship, even with a king? It is speech that conveys what is in the heart. Speech can be used to deceive and other proverbs warn of flattery, but the point here is that one with a pure heart considers his words carefully.
Paul uses this same concept in regard to witnessing: "Conduct yourselves wisely toward outsiders, making the best use of the time. Let your speech always be gracious, seasoned with salt, so that you may know how you ought to answer each person" (Colossians 4:5-6).
Does graciousness characterize your speech? Are people encouraged after speaking with you? Even in disagreements, do they acknowledge that you demonstrate due respect and fairness? Do they respect you?
What is gracious speech? It lacks grumbling and sarcasm. It certainly is void of profanity. It pays sincere compliments readily. It is encouraging. Its focus is on the welfare of others. It flows from a mind that is consciously thinking of the right and good thing to say. It flows from a heart that truly cares to honor God and respect one's neighbor.
Have such a heart; speak with gracious words - you must be a friend of the King.