Paul still really loves the Thessalonians, but they have a few more problems this time. Paul mentioned briefly the Day of the Lord in his first letter, but this time, he spends a lot of time on the topic. From 2:3, I surmise that someone had been along to their church and started telling them that the Day of the Lord (the Second Coming?) had already been and gone, and when they read that paragraph in the first letter, they freaked out. Paul wrote this letter back to correct the mistake, and also to warn them against the despondent idleness they had fallen into after thinking that they’d been left behind.