43 So he brought his people out with joy, his chosen ones with singing. 44 And he gave them the lands of the nations, and they took possession of the fruit of the peoples' toil, 45 that they might keep his statutes and observe his laws. Praise the Lord!
The "why" praise is connected to the "whoness" of God.
What makes praise authentic and not performance is an authentic understanding of who GOD is to you in light of your personal relationship and engagement with GOD. For the Christian, knowing GOD has operated in your life in a personal and grace-filled manner ought to cause praise to arise from both your lips and your life.
"Who" is the Lord to the people of GOD in this text?