For when I preach the gospel, I cannot boast, since I am compelled to preach. Woe to me if I do not preach the gospel! If I preach voluntarily, I have a reward; if not voluntarily, I am simply discharging the trust committed to me. What then is my reward? Just this: that in preaching the gospel I may offer it free of charge, and so not make full use of my rights as a preacher of the gospel. 1 Corinthians 9:16-18
What Paul is saying is that he has no sense of pride and achievement because he faithfully preached the gospel without charge. On the contrary, he really has no choice about preaching the gospel.
I am compelled to preach. In other words,
If I do not preach I am miserable. I have really no choice in this matter. I would much rather preach than experience what I know I am going to experience if I do not: the lash of my conscience, the sense of failure in what God has called me to do. I cannot live with that. Woe to me if I preach not the gospel. He says,
If I do it willingly I gain a reward. If I accept this commission from God, and joyfully do what he tells me to do, it is to my great advantage. I enjoy it; but whether I like it or not, I have to do it.