The psalm begins with these words: "Give ear, O my people, to my law; incline your ears to the words of my mouth." We know this is God speaking, because of the phrase "to my law." God's law is not just a bunch of rules He desires us to obey; His law is Himself, His essence. All the commandments have to do with love if one looks at them closely. His law is describing who He is and how He is. He is not a murderer; He is not a thief; He is not an adulterer, etc. His commandments are telling us to be like Him. God tells us to incline our ears to His words. Hear St. Augustine: "For that man does godly hearken to the law of God, and the words of His mouth, whose ear humility does incline: not he whose neck pride does lift up. For whatever is poured in is received on the concave surface of humility, is shaken off from the convexity of swelling."
If we believe God is telling us, individually, that our hearts are speaking against Him, this humbles us, leading us to repentance and imploring His grace to aid us. If we do not believe this speaks to us, these words are "shaken off from the convexity of swelling"--pride. The prophet, Jeremiah, tells that the heart is most deceptive. We do not know our own hearts, but do we get angry? complain? We can know our hearts a little because "out of the abundance of the heart the mouth speaks." Is everything out of our mouths--and resulting in our actions--for the love of God and neighbor? Is it generosity? May God have mercy on me!
God gave the Israelites the desire of their heart, and that desire of their heart killed many of them. What is really the desire of our hearts? What is my heart really saying?