Albert Barnes Commentary Psalms 135:3

"Praise ye Jehovah; for Jehovah is good: Sing praises unto his name; for it is pleasant." — Psalms 135:3 (ASV)

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.