There's a worth-reading article that I found at "Desiring God" website. Enjoy!
Quest for Joy
Six Biblical Truths
By John Piper January 1, 1995
(c) Desiring God
Did you know that God commands us to be glad?
"Delight yourself in the Lord and he will give you the desires of your heart." (Psalm 37:4)
1) God created us for his glory
"Bring my sons from afar and my daughters from the ends of the earth,... whom I created for my glory" (Isaiah 43:6-7)
God made us to magnify his greatness - the way telescopes magnify stars. He created us to put his goodness and truth and beauty and wisdom and justice on display. The greatest display of God's glory comes from deep delight in all that he is. This means that God gets the praise and we get the pleasure. God created us so that he is most glorified in us when we are most satisfied in him.