1 Chronicles 21:8
And David said unto God, I have sinned greatly.  .  .  .  I have done very foolishly.
Repentance is the only way to peace and happiness and restored communion with the Lord. When we sin we offend Him. We need to make it right by repenting, or taking sides with God against ourselves. David did it. The prodigal son in Luke 15 did it. We must too.