Why do we praise God?
Imagine someone who never grew up being told to say thank you. Such a person would never be really be appreciative of anything he received in life because he would simply expect it to be done. He would have no appreciation for the hard work, sacrifices, or generosity of others. And while others might simply shrug off his lack of appreciation, he missing out on a part of life that comes from appreciating others and being grateful for what we receive.
The Prefaces for the Eucharistic Prayers speaks well when it states:
Priest: Let us give thanks to the Lord our God.
People: It is right and just.
We give praise first and foremost because it is right to praise God's goodness. After all, God could be less merciful and still be perfectly just.
The praise we give to God is partly our just duty to God but also helps to keep us mindful of God's goodness and love.
As the Preface IV for weekdays states:
For, although you have no need of our praise,
yet our thanksgiving is itself your gift,
since our praises add nothing to your greatness,
but profit us for salvation . . .
If we fail to appreciate God's goodness and mercy and fail to grasp his generosity in our lives, we stunt our spiritual growth. We would risk falling into the sin of presumption and losing our salvation.