Tranquility can't be grasped

except by those who have reached an unwavering and firm power of judgment

—the rest constantly fall and rise in their decisions,

wavering in a state of alternatively rejecting and accepting things.

What is the cause of this back and forth?

It's because nothing is clear and they rely on the most uncertain guide - common opinion.

 Seneca, Moral Letters, 95.57b-58a (resource) Ryan Holiday, The 366 Meditations on Wisdom, Perseverance, and the Art of Living—