why we sin

From Rick Gamache's sermon and Desiring God:

We sin because we forget God’s steadfast love and abundant mercy. When we are not ravished by him, we forget the superior pleasures that there are in God and give ourselves to the inferior pleasures of sin. And this is why David says, “Against you God, you only have I sinned.” He goes deep with his confession because he knows repentance is the way back to fellowship with God....
Sexual sin is a symptom of lack of fullness of joy and gladness in Jesus. It’s a symptom of a lack of being ravished by the love and kindness and mercy and goodness and beauty and excellence and majesty and glory and honor and power of God.