Albert Barnes Commentary Psalms 104:31

"Let the glory of Jehovah endure for ever; Let Jehovah rejoice in his works:" — Psalms 104:31 (ASV)

The glory of the Lord shall endure for ever – The margin, as in Hebrew, notes “shall be.” It might be rendered, “Let the glory of the Lord be for ever,” implying a strong desire that it should be so. However, the language may also denote a strong conviction that it would be so.

The mind of the writer was filled with wonder at the beauty and variety of the works of God on the land, in the air, and in the waters. He exclaims, with a heart full of admiration, that the glory of a Being who had made all these things could never cease but must endure forever. All the glory of man would pass away; all the monuments he would build would be destroyed; all the works of art he executed must perish. But the glory of the One who made the earth and filled it with such wonders could not but endure forever and ever.

The Lord shall rejoice in his works – See Genesis 1:31. The idea here is that God finds pleasure in the contemplation of his own works: in the beauty and order of creation, and in the happiness which he sees as the result of his work of creation. There is no impropriety in supposing that God finds pleasure in the manifestation of the wisdom, the power, the goodness, the mercy, and the love of his own glorious nature.