Thankful \ ˈthaŋk-fəl \
conscious of benefit received; expressive of thanks
Have you thought about the fact that God has everything He could ever need? He can do whatever He wants to do. He can have whatever He wants to have. And yet, there is one things we can give Him: our gratitude.
When we are forgiven, when we are rescued from our wretchedness, when we are set free- there is only one proper response: endless, constant gratitude. That's why the Psalmist tells us to enter his gates with thanksgiving: because He has been so, so, so good to us, and the only thing we can give him is our praise, worship, and thanks.
Enter his gates with thanksgiving and his courts with praise; give thanks to him and praise his name. For the Lord is good and his love endures forever; his faithfulness continues through all generations.