Why did Paul write 1thessalonians?

Answer 1

When he converted the Thessalonians, Paul had taught them that the end of the age was imminent and that they were to enter the kingdom when Jesus returned. Now the Christians of Thessalonica feared for those of their congregation who had already died before the second coming of Jesus, so Paul wrote to set the record straight and offer encouragement. He assured them that even those who have died will be brought into the kingdom. In fact, when Jesus returns in glory on the clouds of heaven, "the dead in Christ will rise first; then we who are alive, who remain, will be caught up together with them to meet the Lord in the air" (4:17).
Incidentally, this is believed to be the only letter that Paul wrote to the Thessalonians. The epistle known as 2Thessalonians is regarded by most scholars as a pseudepigraphical work written long after Paul's death. The purpose of 2 Thessalonians was, at least in part, to counter the earlier letter, once it had become clear that Jesus had not returned within Paul's lifetime and was unlikely to return soon. Paul's expectation of an imminent second coming was no doubt becoming embarrassing, so religious leaders needed to show that this was not what Paul really said.

Answer 2

To praise them for their excellent progress.