Why did the Stoics say virtue is the only good?

Most people's working assumption is that health, wealth, reputation, and comfort are goods, and their absence is bad. The Stoics, starting with Zeno of Citium, denied this directly. According to Diogenes Laertius's summary of Stoic doctrine in Lives of the Eminent Philosophers (book 7), the Stoics held that virtue alone is good and vice alone is bad; everything else — health, sickness, wealth, poverty, reputation, even life and death — is “indifferent” (adiaphora) to happiness, because none of it is within a person's control or reliably tracks whether they are living well. The reasoning is that a good, by Stoic definition, is something that always benefits its possessor and can never be misused. Virtue passes that test; wealth doesn't, since a bad person can use it to do harm and a good person's character isn't improved by having it. But the Stoics didn't treat all indifferents as equal to each other. Chrysippus and later Stoics distinguished “preferred indifferents” (health, resources, a good reputation) from “dispreferred” ones (sickness, poverty) — a Stoic sage would still rationally choose health over sickness, all else equal, without regarding that choice as a matter of good and evil. The distinction matters practically: it lets Stoicism recommend engaged participation in ordinary life — earning a living, caring for one's body, pursuing a career — while insisting none of it is what happiness actually depends on. A Stoic who loses their wealth or reputation hasn't lost anything good; they've only lost something preferred. What can't be taken from them, on this view, is the one thing that was ever actually theirs: the quality of their own choices.