As we become soft and lazy in our bodies, we tend to become soft and lazy spiritually. When Paul talked about making his body his slave, so that after having preached to others he himself would not be disqualified, he was not thinking about physical disqualification, but spiritual. He knew well that physical softness inevitably leads to spiritual softness. When the body is pampered and indulged, the instincts and passions of the body tend to get the upper hand and dominate our thoughts and actions. We tend to do not what we should do, but what we want to do, as we follow the craving of our sinful nature.