Psalm 69:29-30 But I am poor and sorrowful: let my salvation, O God, set me up on high. I will praise the name of God with a song, and will magnify him with thanksgiving.
David confesses that he is a poor, miserable sinner in a poor and sorrowful condition. He desperately needs God. Therefore, he responds by praising God with a song and magnifying with thanksgiving .
Whatever situation we find ourselves in, we can still praise the Lord, for He is always good. There is no variableness or turning with Him. When we praise Him with song we feel better and our spirit is lifted. By faith we speak the truth to ourself that He is great and good to us, regardless of how we are feeling at the moment. We carry a song in our heart all the time.
We magnify Him, not to make God bigger than He is, but to strengthen our own faith and perception of God in our minds. We make Him bigger in our minds to stretch our imagination to come closer to the true greatness of God.
We pray to comprehend how high, deep, long, and wide is the love of God toward us. So we thank Him. Every time we thank God He is magnified in our minds and our faith is stronger.