The stoics fundamentally taught that moral substance exists only in intent -- that is, good and evil exist only in willful action. Therefore, it makes no sense to think of external circumstances or things out of our control as being either good or bad. Ownership of things is out of our control, and therefore neither good nor bad. To a stoic, desire is putting moral weight on something you cannot control. You can read more about this here: http://ptypes.com/common-false-values.html
