Let me hear joy and gladness;
let the bones you have crushed rejoice.
When you are coming away from sin, so many times there is no joy and gladness. I know that sometimes there is, but many times as you walk away, you end up more like Lot's wife. Instead of looking forward, you are constantly looking back.
There are some that never get past that part.
I think that this is why David is asking for the joy. He knows it is there, he knows it is coming, but he isn't quite there yet. So he cries out to God, "Let me hear joy and gladness!" God you have crushed me. I know it was for my own good, but I am ready to move on to the rejoicing part!