Psalm 9:1-2
I will praise thee, O LORD, with my whole heart; I will shew forth all Thy marvellous works.
I will be glad and rejoice in Thee: I will sing praise to Thy Name, O Thou Most High.

David knew that it is the duty of the believer to give praise to the LORD Most High.  If our praise is to be acceptable to Him, we must give it from our whole hearts and not only with our lips.  God has done enough where we can always find something to praise Him for, including, but not limited to, the very breath that we are breathing at this moment.  We can praise Him for the sight that we are using to read these words.  We can praise Him for the creature comforts that we enjoy daily.  More importantly than any of that, we should praise Him for His death, burial, and resurrection.  We should praise Him for taking on the penalty of our sin, and for preparing a place for us where we can spend all eternity with Him.  We deserve death, but He has given us life!  For that, we should praise Him.  
All that He has done should pass through the joy that lives in us and manifest itself in our outward praise.

Be glad in Him, believer!  Sing praises to His Name, and give Him the glory He is due.