It doesn’t make sense to thank someone for something they didn’t do. If my wife makes me a sandwich, I might thank her dad for raising her and giving her to me, but I won’t thank him for the sandwich. But Paul thanks God for everything, because God is the ultimate cause of every good thing.
Paul thanks God for the faith, love, and hope of the Thessalonians (1 Thess 1:2-3) because these things show that God chose them (1 Thess 1:4-5). So Paul thanks God that when the Thessalonians heard the word of God, they recognized it as the word of God (1 Thess 2:13). When we see God as the source of every blessing, we will be able to “give thanks in all circumstances” (1 Thess 5:18, ESV).