Psalm 97:1 The Lord reigneth; let the earth rejoice; let the multitude of isles be glad thereof.
Because it is the Lord who reigns over the universe, all the earth rejoices. The islands and coastlands and all far off places are glad because the Lord God is in charge. He rules everything that is created and especially His people whom He has redeemed. He is in control of our lives for our good, from birth to eternity. He exercises His almighty, creative power over the earth for our good, for He is good, kind, and merciful forever.
He, who is above all, is our God. Therefore, with the created universe, we rejoice and are glad. “Everything that has breath praises the Lord.” It is not so much a stretch to believe that our Creator God is all-powerful. Just look at nature. But it does stretch our faith to believe that He uses His power for our good, That He loves us unconditionally and eternally is a little more challenging for us to believe. But by faith we “rejoice in the Lord always.”
It is indeed awful to experience the suffering of sin, and it is horrible to see the suffering and death of Christ for our sin. But underneath it all, we see the joy of salvation. Hebrews 12:2: “Looking to Jesus, who for the joy that was set before him endured the cross.” “The joy” was our forgiveness, life, salvation, and resurrection to life. Therefore, we rejoice, along with all Creation, looking for Redemption.