Non-Christians do experience a measure of joy; the Lord graciously blesses them with the delights of family and friends.
As an example, many American mothers (regardless of their relationships with Christ) are enjoying special attention today as their children shower them with gifts or make them breakfast in bed. For many, Mother’s Day is a joyful celebration.
Christians, however, have an even deeper joy because Jesus has paid for our sins and given us new life. We see His hand in creation, and rejoice that all His works point back to His glory. Even our ability to love each other ultimately comes from Him.
The Lord is the source of all joy, whether we acknowledge Him or not. Christians merely recognize, by His grace, that joy comes from Him. Consequently, we rejoice all the more as we praise Him for filling us with joy divine!