From Rohan Healey's Greeks to Geeks: Practical Stoicism for the 21st Century:
"Of Preferences and Aversion:
I Want Want Want it, but I Don't Need Need Need it!
Epictetus: 'Remember that following desire promises the attainment of that of which you are desirous, and version promises the avoiding that to which you are averse. However, he who fails to obtain the object of his desire is disappointed, and he who incurs the object of his aversion wretched. If, then, you confine your aversion to those objects only which are contrary to the natural use of your faculties, which you have in your own control, you will never incur anything to which you are averse. But if you are averse to sickness, or death, or poverty, you will be wretched. Remove aversion, then, from all things contrary to the nature of what is in our control, and transfer it to things contrary to the nature of what is in our control. But, for the present, totally suppress desire: for, if you desire any of the things which are not in your own control, you must necessarily be disappointed; and of those which are, and which it is laudable to desire, nothing is yet in your possession. Use only the appropriate actions of pursuit and avoidance; and even these lightly, and with gentleness and reservation.'
. . . What Epictetus is saying here is that if you place your preferences and aversions on things outside of your power, you will naturally be unhappy, because your happiness is determined by whether or not you get what you desire, and avoid what you are averse to. And when outside forces are determining whether or not you get what you want, you will, eventually, be disappointed. Therefore, he says, prefer and move away from things within your power only, and reserve a kind of indifference to everything else . . .
It is suggested in Stoicism, in regard to external things, both desirable and undesirable, that they should be approached with a kind of indifference or apathy. This is not a negative indifference or apathy, though these words have gained quite a negative reputation, these are of the positive kind. The Stoic will still move naturally in the direction of that which they like and which is an external 'good' (health, wealth, good relationships, good reputation, etc.), just as they will as much as they can move away from that which they don't like, an external 'bad' (illness, poverty, bad relationships, a poor reputation, etc.). But the key point is that should either fortune or misfortune strike, it will be viewed with the sort of indifference that would be shown to anything outside the Stoic's control. They enjoy good fortune, but no more than it deserves, and they do not take credit for gains acquired from outside forces. Stoics do not reject wealth, power, property, status, or good reputation, they just don't need it. If it comes, it comes, if it doesn't, it doesn't.
. . . If you can stay pretty much the same and at peace in good circumstance and bad you will know that you are a Stoic."
I think Healey masterfully states some of the core ethics of Stoicism in very simple terms, here!