1 Corinthians 9:16-23 – Why do I do this?

If I proclaim the gospel, this gives me no ground for boasting, for an obligation is laid on me, and woe to me if I do not proclaim the gospel! For if I do this of my own will, I have a reward; but if not of my own will, I am entrusted with a commission. What then is my reward? Just this: that in my proclamation I may make the gospel free of charge, so as not to make full use of my rights in the gospel.