“You return man to dust and say, “Return, O children of man” (Ps. 90:3, ESV). Remember the background of this prayer. Yahweh has ordained the death of the rebellious Israelites. He has ordained their “death march” in the wilderness. Just as God has ordained creation, so He has ordained death. Humans live under the divine will of mortality. Thus, human life is so temporary.
“You sweep them away as with a flood; they are like a dream, like grass that is renewed in the morning: in the morning it flourishes and is renewed; in the evening it fades and withers” (Ps. 90:5-6, ESV). Moses admits that God is the cause of mortality. Like a flood that sweeps, God sweeps away humanity. Human lives are like a “dream” (Heb. yashen), “sleep.” How many of you can remember your last dream? We forget our dreams. Our dreams just fade away into nothingness. Moses says, God causes the fading away of people like a dream.
Human life is also like “grass.” In the desert, grass grows in the morning. But it fades in the evening, because of the hot sun. Human life is as short as the desert grass. God has ordained the shortness of life.
You live today because God has allowed you to live today. Your life is short because God has ordained it so. The sovereignty of God can sweep you away like a flood. He can make you pass away like a dream. God can make you wither like the grass. In effect, Moses seems to be saying, “Lord, we have a short calendar.”
What made Moses say that? It is because of the wrath of God against the sin of Israel’s rebellion (vv. 7-8).
 Marvin E. Tate, Psalms 51-100 (WBC 20; ed. John D. W. Watts; Dallas: Word, 1990), 441.