Adam Clarke on Psalms 33:1

Rejoice in the Lord - It is very likely that the last verse of the preceding Psalm was formerly the first verse of this. As this Psalm has no title, the verse was the more easily separated. In the preceding Psalm we have an account of the happiness of the justified man: in this, such are taught how to glorify God, and to praise him for the great things he had done for them.

Praise is comely for the upright - It is right they should give thanks to Him, who is the fountain whence they have received all the good they possess and thankfulness becomes the lips of the upright.