What does Psalm 51:12 mean?

It has been said that sin "causes the cup of joy to spring a leak." It certainly drained David's cup of joy. He urgently asked God to restore the joy of his salvation. Adultery and murder had caused David to lose his joy and become depressed. Only God could restore the lost joy. It is far better to obey than to deliberately disobey, planning to later seek remedial help.

Jesus promised His abiding joy to those who keep His commandments. He said, "If you keep my commandments, you will abide in my love, just as I have kept my Father's commandments and abide in his love. These things I have spoken to you, that my joy may be in you, and that your joy may be full" (John 15:10–11). In addition to David's request for restored joy, he asked God to uphold him with a willing spirit. Such a spirit would keep David committed to God's will and prevent him from falling into sin again.