This instruction about laughter is a difficult one to take seriously. Since when is laughter an enemy of the soul? But the tenth degree of humility is not simply "to not laugh." It is to "be not ready or quick to laugh." I imagine a spring-loaded toy, compressed and waiting for the push of a button to release a bust of energy. The instruction here is to keep the load off the spring. The tenth degree seems to follow from the ninth in that the loaded spring of ready and quick laughter can be another way we impose ourselves upon silence and compromise our hospitality to Spirit.
It is helpful for me to take a step back and think of this instruction as a part of the series of steps in Chapter 7. The intention of the steps, the rungs, is to cultivate a posture of humility in the soul. Humility is of central importance to our Father Benedict because it is the condition of the human soul through which God can speak and act in the world. Each rung on the ladder of humility, it seems to me, dismantles some egoic barrier to this flow of Divine action into and through our lives.
Furthermore, laughter is often put to uses other than a pure expression of delight. Habitual laughter can serve to guard our conversations from ever going beyond the polite. Such laughter can belittle or trivialize, which enables us to avoid honest, vulnerable engagement, and without honesty and vulnerability there is no opening to Spirit.