New Testament scholar P.T. O'Brien notes that “Paul mentions the subject of thanksgiving in his letters more often, line for line, than any other Hellenistic author, pagan or Christian.” For example, in the letter of Colossians alone we have the following, "We always thank God" (1:3), "Giving thanks to the Father" (1:12), "Abounding in thanksgiving" (2:7), "And be thankful" (3:15), our singing is to be with "thankfulness in our hearts to God" (3:16), and Paul exhorts us to do everything every day "giving thanks to God the Father through him" (3:17).
Why so much thanksgiving? Those that know what they deserve and the grace that has come to them through the cross of Christ are compelled to be thankful. The knowledge of our sin and God's mercy are the keys to the heart abounding with thanksgiving. It is also a crucial key to joy. This is how Paul knew himself. To Timothy he expressed his true sentiments. "Christ Jesus came into the world to save sinners, of whom I am the foremost" (1 Tim 1:15). Is that how we see ourselves?
It is the key to gratitude, thanksgiving, and joy.