Why did Paul keep his body under control?
Paul kept his body under control lest it became an instrument of unrighteousness. The constant admonition in his letters is that believers should not yield their members as instruments of sin or give in to old Adam, but keep him under subjection. This is an exercise of will and yielding, not torture.
Les emphasizes the exercise of will in the believer’s life, because the word does not demand but calls for yielding. Paul says, do not yield or give in and let your members be instruments of unrighteousness unto sin or again unto old Adam. The constant admonition of Paul’s letters to the believer is don’t give in to old Adam, keep him under subjection. Paul had to do it in himself. He says back in Corinthians that he kept his body under control, lest it became an instrument of unrighteousness. That does not mean becoming a cleric, a monk in a monastery, or torturing oneself. The Christian life is the greatest life of freedom, joy, and responsibility; we do not have to subject ourselves to self-torment to be spiritual.