Paul had only preached the Good News and ministered to the Thessalonians for a few weeks before he had to flee (Acts 17:1-10). Later, the believers had some questions either to clarify or as a reminder of what Paul taught them. This is part of the greeting to his (and our) fellow believers.
My translation and notes.
1 Thessalonians 1:2-4
2 We always thank God for all of youp, mentioning youp in our prayers.
3 Without ceasing we remember yourp work of faith, labor of love, and perseverance of hope in our Lord Jesus Christ, in the sight of God and our Father,
[These deeds mentioned can be physical toil or spiritual toil or both. When we do our work for the honor and glory of God (1 Corinthians 10:31), there is a blessing to all. There are no threats from our Lord. We work, labor, and persevere because we love God. Be faithful. Compare Hebrews 6:10, Luke 17:10, Revelation 2:2, and 1 Corinthians 16:14. Barclay (Daily Study Bible) has this note: Bernard Newman tells how once he stayed in a Bulgarian peasant’s house. All the time he was there the daughter was stitching away at a dress. He said to her, “Don’t you ever get tired of that eternal sewing?” “O no!” she said, “you see this is my wedding dress.” Work done for love always has a glory].
4 Knowing, brethren loved by God, [his] choice of youp.
[God’s love is the love of choice. God loved us before we loved Him. See John 3:16 and 1 John 4:19. Why did He choose us? God is love (1 John 4:8). His love cannot be secret. It is shown and demonstrated through words and deeds. What can be a greater love than to love enemies (Romans 5:8)? How did God choose? Was it His mysterious will to choose a few and neglect the others? Was it because the Holy Spirit opened everyone’s mind and heart, freeing their will, so they could believe the Gospel? See Acts 16:12-15, John 16:8-11, John 12:48 (how can one persistently reject Jesus and His message? They make a choice), 2 Corinthians 4:3-4, John 6:44 (God draws (literally drags) us to Him (by love), but not choosing for us predetermining who would and who would not), John 3:36 (good choice or bad choice, opportunity versus predetermined), 1 Timothy 4:10 (the Savior of all humans but particularly those who believe, so salvation is provided but most refuse). Why do they refuse? People refuse to believe because of pride (“I’m good enough—Psalm 10:4) and/or love of sinful pleasures (Revelation 9:20-21). All humans were loved so God provided the Good News. Those who believe are loved for they are those who obeyed the Good News (1 John 3:1, Ephesians 5:2, and 1 Corinthians 2:9].