What does 1 Thessalonians 5:18 mean?

In this verse Paul exhorts his readers to give thanks in all circumstances. He adds that it is God's will to do so. Regardless how difficult our circumstances may be, we can find reasons to thank God. We can be thankful that all things work together for our good if we belong to HIm (Romans 8:28). We can be thankful that God's grace is sufficient to sustain us in every circumstance (2 Corinthians 12:9). We can be thankful that nothing can separate us from God's love (Romans 8:37–39). We can be thankful that God even supports us spiritually when trials produce difficulty in our prayer life (Romans 8:26). Rough circumstances also prove the genuineness of our faith (1 Peter 1:6–7).

Paul previously wrote to "Rejoice always" (1 Thessalonians 5:16). Joy is not the same thing as happiness. In a similar way, Scripture notes the difference between giving thanks "for" our circumstances, as opposed to giving thanks "in" our circumstances. For example, a car wreck may not find us giving thanks for a twisted fender and smashed radiator, but we can give thanks that we have insurance and a God who is always present with us. We may not thank God for an illness, but we can be thankful for medicines and doctors. Persecution is not something we would be thankful for, but we can be thankful that God can use that hardship for our eternal benefit.