Sermons On The Second Reading
If you can't refute the argument, then you can attack the person, and the best way to attack a person is to question the motives. So Paul is responding to attacks upon the Good News of Jesus Christ by those who have attacked him and questioned his motives for coming to Thessalonica. He rejoices that when he came to preach, people heard the message joyfully. Paul says he preaches because he has to. He preaches to please God. But, of course, there were some who suggested that Paul was really preaching from some other reason. He was preaching in order to enhance his own self-interest.