The first discipline concerns what someone striving for excellence as a rational being should truly believe is worthy of desire, which for the Stoics is that which is truly good, virtue and action motivated by virtue.
Of these [three areas of study], the principle, and most urgent, is that which has to do with the passions; for these are produced in no other way than by the disappointment of our desires, and the incurring of our aversions. It is this that introduces disturbances, tumults, misfortunes, and calamities; and causes sorrow, lamentation and envy; and renders us envious and jealous, and thus incapable of listening to reason. (Discourses 3.2.3, trans. Hard)
Epictetus remarks: 'When I see a man anxious, I say, What does this man want? If he did not want some thing which is not in his power, how could he be anxious?' (Discourses 2.13.1, trans. Long). Those things that most of us, most of the time, seek after as being desirable, what we consider will make our lives go well, are things that are not in our power, and thus the hope we have for securing these things is placed in the hands of others or in the hands of fate. And when we are thwarted in our efforts to gain what we desire we become frustrated (or depressed or envious or angry, or all of these things). To be afflicted with such 'passions', says Epictetus, is the only real source of misery for human beings. Instead of trying to relieve ourselves of these unpleasant emotions by pressing all the harder to secure what we desire, we should rather place our hope not in 'external' things that are not in our power, but in our own dispositions and moral character. In short, we should limit our desire to virtue and to becoming (to the best of our capacities) examples of 'excellence'. If we do not do this, the inevitable result is that we will continue to desire what we may fail to obtain or lose once we have it, and in consequence suffer the unhappiness of emotional disquiet (or worse). And as is the common experience of all people at some time or other, when we are in the grip of such emotions we run the risk of becoming blind to the best course of action, even when construed in terms of pursuing 'external' things.