Notice the attitude with which Paul presents his message here: He says that he had a need to preach the gospel, that "necessity is laid upon me." He is saying, "I've got to preach this. And if I don't, I'd better hide under a rock somewhere." His approach to preaching the gospel was that lightning could strike at any time if he stopped preaching it. Remember how Jesus started this apostle's ministry—by blinding him out in the middle of the road to Damascus! What Jesus did made quite an impression on him. Paul probably thought about that often—and about what would happen if he stopped doing the charge that had been laid upon him.
So, a true minister feels a compulsion to preach the gospel. On the flipside, he feels a certain doom if he does not. He feels as though, if he tried anything else, he would be a fish out of water. In him would be an emptiness that was not being filled because the ministry of Jesus Christ is a calling. To a true minister of God, it is not just a job done for a paycheck, but a vocation that he feels compelled to do because the truth must be preached. The information cannot remain in his mind. Uncommunicated knowledge and understanding of God's way is absolutely useless to anybody else; it cannot help anybody take even one step along the road to the Kingdom of God. This is why God gives ministers mouths to speak.
Paul calls it "the foolishness of preaching" (I Corinthians 1:21, KJV), but God accomplishes a great deal through it. The serious, devoted servant of Jesus Christ must do it. He just must! It is almost like a man who has just walked out of the desert and has not had anything to eat or drink. He feels compelled, obviously, to find something to satisfy his need—particularly to drink. A true minister of God feels this way if he has not preached the truth in a long while. A "pressure" builds up after a time, and if it is not released, it explodes. The truth must be passed on because a compulsion from God Himself drives a true minister to speak the truth.
This may be dramatizing it a bit, but there is a feeling in a true minister's gut to let other people know the truth that he has been given to preach. In a true minister of God, the truth will out, to borrow a line from Shakespeare.
On the other hand, what does the minister get out of it? What does Paul say that he got from it? In verse 18, he says that, when he preached the truth, other people were helped. He was able to preach the gospel in love, without charge. He did not need anything for it personally because he was driven from the inside—not the outside. Thus, it made him feel good—it relieved that God-sent pressure and gave him the satisfaction that he had done his job.
Richard T. Ritenbaugh