For Christ did not please Himself, but, as it is written, “The insults of those who insult you have fallen on me.” Romans 15:3
Psalm 69:9 is quoted by Apostle Paul because it directly relates to our Lord Jesus Christ. For, there are those who insult and mock God and those insults are absorbed by Christ to bring harmony. As Paul continues in his discourse about harmony, he is helping us to understand that through your response to others, with your steadfast faith in Jesus, you start and sustain harmony with other people.
We cherish harmony and good will toward each other for it is a blessing from our Lord in our relationships.