Our God is special. He does things hardly anyone expects though He expects everyone will notice. Paul told the Romans that all men know God but they won’t honor Him or give thanks to Him (Romans 1:21). It strikes me that gospel, the power of God to salvation, goes after the ungrateful. The good news that causes us to be most thankful addresses sinners who are the least thankful.
Salvation, analyzed in one way, is deliverance from ungratefulness. God justifies unthankful men, forgiving them and declaring them innocent of all their thanklessness. God sanctifies men by the gracious work of His Word and Spirit to make a man more thankful. Even the word itself expresses a great measure: thankFUL, or grateFUL. A saved man is full of it, in the right way.
So there are two parts to this exhortation. First, is your thank tank full? If not, you should confess it as sin.
Second, are you faithfully reflecting God to the unthankful around you? Jesus told His disciples,
love your enemies, and do good, and lend, expecting nothing in return, and your reward will be great, and you will be sons of the Most High, for he is kind to the ungrateful and the evil. (Luke 6:35, ESV)
Using unthankfulness to fix an unthankful neighbor works as well as using a Brillo pad to fix a scratch on his eyeball. If we obey the command to “give thanks in all circumstances” (1 Thesslalonians 5:18), we may find that thanks is potent to overcome complaints. We won’t overcome evil with grouchiness because God doesn’t.