Epictetus, the former slave and first-century Stoic philosopher, taught that freedom is an inner quality, not a quality of circumstance, and that life should be lived with a tendency toward personal achievement. In light of these beliefs and teachings--remembering that he discusses freedom from a slave's vantage point--he divides events and circumstances into two kinds: those over which we have certain control and those over which we have uncertain--or no--control. In conjunction with this idea, he likened life to a children's game and that the unfortunate thing about the game was that there were so few winners. [More details are available in an article from World Philosophers and Their Works.]
Therefore, when Epictetus says that things that are up to us--the direction of effort, the outcome, etc--are by nature free, he is reinforcing his tenet that freedom is an inner quality. For things that are up to us come from inner reasoning, choice, effort, etc., thus are by nature free and therefore tend toward personal achievement. Conversely, when he says that things that are not up to us, e.g., acceptance at a university, award of an employment position, receipt of a bank loan, are weak and enslaving, he is reinforcing his tenet that it is right to maintain an attitude of indifference toward things over which we have no control, as the children's game of life has--in the main--few winners, i.e., no one goes unscathed by trouble and suffering through life.
This relates to desires in the following ways. If we strongly desire inner freedom, as explained above, then we may attain some varying measures of personal achievement. Therefore such desire is by nature free. On the other hand, if we strongly desire that university, that job, that bank loan, we become personally weakened by the possibility (and sometimes the reality) of denial and are thus enslaved by our desire and by the control others in reality have over us in such circumstances. Therefore such desire is weak and enslaving. Epictetus's solution is to maintain a stoical attitude of indifference toward things over which others or external circumstances exert control.