"And now, Lord, what do I wait for? My hope is in You."
In vv. 4-6, David had surveyed his life, and determined that there wasn't really much to it, especially time-frame-wise. Now, it's as if he says to God, "In light of the brevity of my existence, what does it all amount to? What am I here for?" And the answer, of course, is, "I am here for You, Lord. You are the only One I hope in."
What do we "wait for"? If it is anything less than the Lord Jesus Christ Himself we are wasting our lives, and our days. Let us put our hope in Him. Let us "wait" for Him.
[Puritan quote of the day: "True [gospel] mourning begins in the love of God, and ends in the hatred of sin." --Thomas Watson, in "The Beatitudes"]