Jamieson, Fausset and Brown on 1 Corinthians 9:16

16. though I preach ... I have nothing to glory of--that is, If I preach the Gospel, and do so not gratuitously, I have no matter for "glorying." For the "necessity" that is laid on me to preach (compare Jr 20:9, and the case of Jonah) does away with ground for "glorying." The sole ground for the latter that I have, is my preaching without charge (1Co 9:18): since there is no necessity laid on me as to the latter, it is my voluntary act for the Gospel's sake.