The author is willing to do God’s will. God’s law is in his heart. The core of his existence is defined by God’s laws; he will heartily obey. We hear the echo of the “shema” from Deuteronomy 6:1–9 (shema being the Jewish prayer that served as the centrepiece of the morning and evening Jewish prayers). You may also think of the song of praise for the law, in Psalm 119:1–176.
In Hosea 6:6 we learn of a slightly different contradiction than sacrifices versus obedience, namely sacrifices versus love. That is also knowing God: “For I desire steadfast love and not sacrifice, the knowledge of God rather than burnt offerings.” Paul describes his inner struggle in Romans 7 and confesses his love for the law in Romans 7:22–25: For I delight in the law of God, in my inner being…captive to the law of sin…Wretched man that I am…Thanks be to God through Jesus Christ our Lord!
The devoted apostle understands the spiritual battle, but then also with rest: Jesus Christ the Saviour.
8 I delight to do your will, O my God; your law is within my heart.”