Praise the Lord, for the Lord is good - See Psalms 107:1.
Sing praises unto his name, for it is pleasant - See Psalms 33:1; Psalms 92:1. The idea here is, that it is a source of happiness, and that it is proper in itself.
Related Commentaries of Psalms 135:3
Psalms 135:3
3 Praise the LORD; for the LORD is good: sing praises unto his name; for it is pleasant.