The Lord has always been (Psalm 90:2), but man heads to destruction (Psalm 90:3), because, 

“You carry them away like a flood; they are like a sleep. In the morning they are like grass which grows up: in the morning it flourishes and grows up; in the evening it is cut down and withers” (Psa 90:5-6).

 
Man might even live as long as seventy or eighty years, and sometimes even beyond, but it still falls far short of the eternity of God. Therefore, we want Him to teach us how to live, how to make the most of our days.