Why do spiritual exercises? Jesus.
It's all about Jesus Christ. We keep the end in sight, the fruit of our discipline. Workouts improve the game, studying improves the citizenship and parenthood. Spiritual exercises - because we need Jesus, the only and glorious option. The hope increases as we near heaven. It never fades; we count all else as loss. Paul sets an extreme example - by human standards, he seemed pretty perfect. Yet only Jesus matters to him.
Christ is our life. He is our Righteousness, for we have none of ourselves. In this we are content, whatever else we have - sickness or health, poverty or riches, death or life. For our citizenship is in heaven. We have Jesus Christ, the only eternal hope. Minds, bodies, and possessions fade, but Jesus does not. He is why we discipline ourselves spiritually. Grow in the grace and knowledge of our Lord Jesus Christ. All else is loss. He is with you!
Soli Deo gloria.