The words "every day" should characterize the times we spend praising the Lord. Unfortunately, we often get so wrapped up in the chores of the day and we fail to stop and praise God the way we should. David is not calling on us to praise God every moment of the day; rather he is asking us to make it part of our lives by considering the reasons to be grateful to God for the life He has given us. Let's suppose we are having a real difficult time and can't think of any reason to praise God. At the least, we should thank Him for giving us life and for the moment He has given us to praise Him.
God knows that the kind of person willing to show gratitude to Him is also the kind of person willing to show gratitude to others. Expressing gratitude and thanksgiving makes us a more joyful person. David was going through some really tough times but he was not exempt from praising God and others around him needed to experience that praise in their life also.
 Great is the LORD, and greatly to be praised; and his greatness is unsearchable..
We have already seen God is to be praised no matter what the circumstance. Now notice the last part of verse 3 that reminds us that no one can fathom how great He is to us. We can never fully comprehend God's power, His presence and His knowledge. God made many promises to David about the future kings that would be his descendants. David's descendants for hundreds of years ruled over parts of Israel. Although this may be difficult for David to comprehend at the moment, he needed to believe God by faith and praise Him.
God is worthy of praise if for no other reason, than He has tremendous power that is beyond our ability to comprehend. God is not asking us or expecting us to understand the extent of His power. God is just asking us to trust Him with our lives even if we can't fully understand Him.