“For if a person shifts their caution to their own reasoned choices and the acts of those choices, they will at the same time gain the will to avoid, but if they shift their caution away from their own reasoned choices to things not under their control, seeking to avoid what is controlled by others, they will then be agitated, fearful, and unstable.” —EPICTETUS
...Epictetus is reminding you that serenity and stability are results of your choices and judgment, not your environment. If you seek to avoid all disruptions to tranquility—other people, external events, stress—you will never be successful. Your problems will follow you wherever you run and hide. But if you seek to avoid the harmful and disruptive judgments that cause those problems, then you will be stable and steady wherever you happen to be.