Why write a psalm? Why not? My reasons are:
God is worthy of thanks, praise, and worship. Writing and reciting or singing a psalm is one way to give God the honor He deserves.
We are made to worship God, so entering into true worship of our loving Father gives us joy and peace.
Praising God is a good way to take my mind off of the multitudes of problems we have in this world, and remind myself of Jesus’ victory over the world.
Honoring God with thanksgiving, praise, and worship really annoys the enemy and is therefore a good way to resist him.
God likes it. (In my case, I think it is kind of like the way parents post their young children’s art on their refrigerator. He likes it more because He loves me than because of my talent.)
Ephesians 5:19 and Colossians 3:16 encourage speaking to one another with psalms, hymns, and spiritual songs.
It is fun.
People who read psalms tend to be encouraged.
Psalm-writing helps you exercise and build up your faith.
Psalms make great ammunition in spiritual warfare.
Sometimes my spirit burns within me in a zeal to praise God and/or proclaim His Word, and psalm writing is a constructive way to deal with that burning desire.
It might encourage other people to write psalms, so that God gets even more glory.
Try it. You’ll like it. Study how the psalms in the Bible are written, meditate on them, and ask the Holy Spirit to help you. He is delighted to help people bring more glory to the Father and the Son!