The Thessalonians were a church, like us under pressure to conform to the ways of society. Paul is writing to encourage them to not do that, and instead to dedicate themselves to living holy lives for God. Lives that set them apart from the world around them. Why should we do this? Well, for Paul, and for us, the reason we should do this is that Jesus will return. Which is what makes 1 Thessalonians a good book for Advent.