Answer:
David fasted out of repentance and as part of his appeal to God to relent of His sentence regarding the child. Once David received God's answer, there was no reason to continue the fast. Yet his servants are baffled. They cannot understand how David could appear so grieved before and be less so now. But it's not that David is no longer grieved. It is simply that he no longer needs to fast. The fast was never about punishing and abusing himself-to satisfy God with substitute punishment-or to obligate God to fulfill his request.