But let all those rejoice who put their trust in You;
Let them ever shout for joy, because You defend them;
Let those also who love Your name
Be joyful in You.
For You, O Lord, will bless the righteous;
With favor You will surround him as with a shield .
In verse 11 " Let all those rejoice who put their trust in You" It's not their words, but trusting in the Lord that brings hope and joy no matter what. They rejoice, they shout for joy, and they are joyful in the Lord. I've known the greatest joy in my life in the darkest tunnels. God has shown up with unexplainable joy to strengthen me (check out Unexplainable Joy ).
This Psalm calls us to joy. "God promises joy and gladness to believers. Light is sown for them: the Lord will turn their night into day." Spurgeon