Because God is truly glorious,
Psalms 115:1 Not unto us, O LORD, not unto us, but unto thy name give glory, for thy mercy, and for thy truth's sake. Psalms 115:2 Wherefore should the nations say, Where is now their God? Psalms 115:3 But our God is in the heavens: he hath done whatsoever he pleased.
and idols are vanity,
Psalms 115:4 Their idols are silver and gold, the work of men's hands. Psalms 115:5 They have mouths, but they speak not; eyes have they, but they see not; Psalms 115:6 They have ears, but they hear not; noses have they, but they smell not; Psalms 115:7 They have hands, but they handle not; feet have they, but they walk not; neither speak they through their throat. Psalms 115:8 They that make them shall be like unto them; yea, every one that trusteth in them.
he exhorteth to confidence in God.
Psalms 115:9 O Israel, trust thou in the LORD: he is their help and their shield. Psalms 115:10 O house of Aaron, trust ye in the LORD: he is their help and their shield. Psalms 115:11 Ye that fear the LORD, trust in the LORD: he is their help and their shield.
God is to be praised for his blessings.
Psalms 115:12 The LORD hath been mindful of us; he will bless [us]: he will bless the house of Israel; he will bless the house of Aaron. Psalms 115:13 He will bless them that fear the LORD, both small and great. Psalms 115:14 The LORD increase you more and more, you and your children. Psalms 115:15 Blessed are ye of the LORD, which made heaven and earth. Psalms 115:16 The heavens are the heavens of the LORD; but the earth hath he given to the children of men. Psalms 115:17 The dead praise not the LORD, neither any that go down into silence; Psalms 115:18 But we will bless the LORD from this time forth and for evermore. Praise ye the LORD.