'Restore to me the joy of Your salvation... Then... sinners shall be converted to You.' Psalm 51:12-13
Knowing the gospel is not enough. Evangelism doesn't work when we are satisfied with knowing the gospel. People need to see that we enjoy the gospel. They need to see that we've fallen in love with a beautiful God through the gospel. The attractiveness of Christ is the substance of our joy in God. If the world can't see that Christians are delighted with the gospel they will be forced to assume that Christ is not attractive. And if Christ is not attractive He doesn't need to be and shouldn't be worshipped.
That's why David seeks his happiness in God: 'restore to me the joy of your salvation'. Sinners are converted when Christians rejoice in salvation. Sinners see the beauty of Christ through the joy of Christ's people.