Because God is truly glorious,
Psalms 115:1 Not to us, O LORD, not to us, but to thy name give glory, for thy mercy, [and] for thy truth's sake. Psalms 115:2 Why should the heathen say, Where [is] now their God? Psalms 115:3 But our God [is] in the heavens: he hath done whatever 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 are like them; [so is] 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 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 will increase you more and more, you and your children. Psalms 115:15 Ye [are] blessed of the LORD who made heaven and earth. Psalms 115:16 The heaven, [even] the heavens, [are] the LORD'S: 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 ever. Praise the LORD.