The heading says this is "A Prayer Of Moses, The Man Of God." It is evident as you read this Psalm that he had a depth of experience with both God and man.
Moses begins by declaring the LORD to be our dwelling place. Acts 17:28 says the same thing, "In him we live and move and have our being." Then Moses compares God and man. Verse 2 declares God to be from everlasting. He is the One who formed the earth and brought forth the mountains. Verses 3-6 describes man to be just the opposite. Man is returning to dust just as God promised in Genesis 3:19, "For you are dust, and to dust, you shall return." To liken man to God is to equate the eternal with grass that fades and withers. Not only is man's life short, but his iniquities and sins are always before God. Man justly deserves God's anger and wrath—life is toil and trouble.
That being true, it is easy to see why Moses would request of God a wise heart so that we would correctly number our days and live accordingly. Moses concludes his prayer by expressing His desire for God to have pity on them and exhibit His steadfast love so that they may live their days in rejoicing and gladness.
Any time you notice repeated words, phrases, or ideas, it is worth your time and effort to examine why the author is doing so. Psalm 90 is an excellent example.
First, notice the use of the word “return” in verses 3 and 13. The seeming hopelessness of man returning to dust is countered by the great hope of God returning to set things right. Just as the morning grass is renewed and flourishes before fading away (vs. 5-6), when God returns, He will satisfy us in the morning (v14) with His steadfast love. In the same way, man's seventy to eighty years end in a sigh (vs. 9-10) and like a dream God sweeps them away (vs. 4-5). But the God who is from everlasting to everlasting can make us glad as many years as we have seen evil.
Moses ends by comparing the work of man to the work of God. Interestingly, the "work" of God in verse sixteen is a different word than the "work" of man in verse seventeen. In both instances, the word means to toil or labor. One of the Hebrew language dictionaries I use distinguishes the word in verse sixteen as the work or toil of a maker/creator. Isaiah 5:12 helps us understand the difference, "They have lyre and harp, tambourine and flute and wine at their feasts, but they do not regard the deeds of the LORD or see the work of his hands.” We see a similar distinction in Philippians 2:12-13, "Work out your own salvation with fear and trembling, for it is God who works in you, both to will and to work for his good pleasure." Paul is using the word “work” in the same way as in Psalm 90.
LORD, You are our dwelling place. Only in You do we live, move and exist. We want You to make us wise as we live the days You give us. You alone know both our beginning and our end. Who are we that You would show such love, pity, favor, and glory? Thank You, Jesus, for the work You do in our lives.