First, I should be thankful for thankfulness because it's God's will for me to be thankful. In I Thessalonians 5 Paul is giving the church a list of responsibilities to fulfill. In verse 18 he writes, "give thanks in all circumstances; for this is the will of God in Christ Jesus for you." It's God's will that we give Him thanks in all things. We spend so much time trying to discern God's will that we overlook specific aspects of that will that scripture shows us. It's God's will that I give thanks in all things. That means, then, that when I have a thankful heart I'm walking in His will. So a heart full of gratitude is a blessed grace from God--it is a concrete proof I can look to, a fruit that shows me I'm walking in God's will for my life.
Second, I should be thankful for thankfulness because a thankful heart is a heart that is surrendered to Jesus. In Colossians 3 Paul is outlining what the new self looks like. The gospel says that when Jesus saves us He recreates us into His image (2 Cor. 5:17). The change in who we are will lead to changes in what we do. And Paul is saying, "This is what those changes ought to look like." In verse 15 he says, "And let the peace of Christ rule in your hearts, to which indeed you were called in one body. And be thankful." Notice how he links thankfulness with letting the peace of Jesus rule in our hearts. A surrendered heart is a thankful heart. It's a heart that has learned to trust in and rest on Jesus for all things. That doesn't mean we're happy when bad things happen; means we rejoice that we serve a God who is bigger than those circumstances and is able to work them out for His glory. Paul goes on to write, "Let the word of Christ dwell in you richly, teaching and admonishing one another in all wisdom, singing psalms and hymns and spiritual songs, with thankfulness in your hearts to God. And whatever you do, in word or deed, do everything in the name of the Lord Jesus, giving thanks to God the Father through him." Notice how thankfulness undergirds everything else. A heart that is surrendered to Jesus will bear the fruit of thankfulness. If my heart is bearing that fruit I can give God the glory because He is reminded me that He who began this good work will complete it; and He is in the process of completing it right now. My thankful heart is evidence of the gospel at work in my life. It reminds me that I'm continuing to be conformed, by the grace of God, to the image of Jesus. And that, my friends, is something for which we can be thankful. What a joy to be reminded that the gospel that saves me is the gospel that is changing me. And what a blessing to be able to see the evidences of that change. Not so I can pay myself on the back, but so I can give thanks to One who is doing the work in me.