Jack feels that his workouts are too easy. he should increase the intensity of his exercise program completely change the type of workout he is doing change his mindset stop exercising because he has reached his fitness goals

The answer is : Increase the intensity of his exercise program 
After doing a same intensity exercise over a period of time, our body will adjust to it and the workout will became easier for our body to handle. In order to keep increasing his physical capabilities, jack has to keep increasing his workout intensity like adding more resistance or increasing his workout speed