There are two curious contradictions that become evident after examining the tenets of Stoic philosophy. First is that a sense of free will is somehow maintained despite being in the face of an overwhelming fatalism—how is it that anyone can have any impact on the direction that he takes, if he is condemned to follow the path that fate has preordained for him anyway? Second is the problem of how individualism can be maintained at all (as the Stoics seem to believe it can be), if in our every action we are to be thinking universally (as the Stoics maintain that we ought).
The example that Hippolytus gives (citing Chryssipus and Zeno) of a dog tied to a moving wagon and forced to follow it wherever it goes is useful for elaborating upon the first problem and what it entails. Pulled behind the wagon, the dog has two choices: he can follow along behind it under his own steam, "so that his own power and necessity unite"; or he can struggle against the motion of the wagon and nevertheless be dragged along behind it anyway, despite his best efforts (SVF II, 975). Necessity guides human activities in this way, too; for the Stoic philosopher, regardless of whether it is our will to do so, we "will be absolutely forced to enter into the fated event" (SVF II, 975)—we can struggle against it or go along with it peacefully, but either way we will be obliged to follow the path that we are fated to take. But if we are bound in this way, how does that leave any room whatsoever for rational freedom of will?
It is integral to Stoic philosophy that the soul is governed by reason; the rational part of the soul is "the ruling part, which produces presentation, assent, sensation, and desire" (SVF II, 836a). The soul is a passive subject, acted upon by and reacting to its object—in the case of the fated event toward which a human being is inevitably progressing, it is that necessity itself that is the active object to the human's passive subject. And it is the qualities of soul outlined above—that is, presentation, assent, sensation, and desire—that allow for free will: the freedom is not in deciding whether or not the fated event will occur, but in deciding how to react to its inevitability.
To give one's assent to necessity is to go along with it; in the example of the dog, the choice to follow along behind the cart to which it is tied instead of being dragged. It is obvious in this example that the best decision is to give assent to necessity, as to do so saves the dog in question a lot of inconvenience and discomfort; but in the case of human activities, which choice is best is an issue that at first glance does not have quite as clear an answer.
To clarify this point it may prove valuable to examine what necessity means to the Stoic philosopher. Quoting Chryssipus, Theodorotus said that "that which is compelled by necessity does not differ from that which is fated, for fate is an ordered, continuous, eternal motion" (SVF II, 916a). All things proceed in a fated way from their antecedent causes, whose existence and effects were also fated, etc., and so it goes for all eternity. In other words,
[Fate, or necessity, is] an order and series of causes wherein cause is connected to cause and each cause of itself produces an effect. That is an immortal truth having its source in all eternity. Therefore, nothing has happened which was not going to happen, and, likewise, nothing is going to happen of which nature does not contain the efficient causes. Consequently, we know that fate is that which is called, not superstitiously, but scientifically, "the eternal cause of things, the 'wherefore' of things past, of things present, and of things to come." (SVF II, 921)
It could be argued that we are not bound by fate as the Stoics would have us believe, and that in fact we can choose to ignore necessity altogether and do something that was not fated instead. But in that case, surely something would have caused that decision—and if that something is preceded by a cause (which cause itself is preceded by innumerable causes), how then does that differ from doing that which has been fated? That is, "if anything happened without an antecedent cause, it would be false to say that everything happens by fate; but if it is likely that for everything which happens a cause precedes, what reason can be given why we should not admit that everything occurs by fate?" (SVF II, 974)
With that in mind, following necessity is certainly the inevitable choice, since antecedent causes have lined up to produce effects leading to one's doing just that for all eternity. And giving assent to necessity is the rational choice, for the same reason: being content with one's fated lot (since one's lot is indeed fated) is a better option than being unhappy with it, since either way nothing will change. On this subject Epictetus advises, "[a]sk not that events should happen as you will, but let your will be that events should happen as they do, and you shall have peace" (Epictetus, 8).
There is another reason why giving assent to necessity (or fate) is the best option, and it has to do with what guides fate in the first place—the ultimate cause. For the Stoic philosopher, there is "only one cause, i.e., the Maker"; "[the] collection of causes, as defined by Aristotle and by Plato, [comprehends] either too much or too little" (SVF II, 346a). There is no cause other than "Creative Reason, i.e., God" (SVF II, 346a). With God, Creative Reason, as the only real cause of everything (with all other causes proceeding from it), God also becomes synonymous with necessity. This is reflected by Plutarch, who writes that "[n]othing... either rests or is moved otherwise than according to the reason of Zeus, which is the same thing as fate" (SVF II, 937c). Thus assenting to necessity is the same as aligning one's will with the will of the divine—surely then it is the best decision from an ethical standpoint, too. Cleanthes expresses the Stoic position on free will, necessity, and the divine most aptly and succinctly in the following poetic fragment:
Lead me, O Zeus, and lead me, Destiny,
Whither ordained is by your decree.
I'll follow, doubting not, or if with will
Recreant I falter, I shall follow still. (Cleanthes quoted in Epictetus, 53)
A consequence of thinking of oneself in this way (i.e. as something manipulated by fate) is that we also must think of everything in terms what it is generally, not in terms of its specific importance to us. Epictetus advises, "When anything, from the meanest thing upwards, is attractive or serviceable or an object of affection, remember always to say to yourself, 'What is its nature?'" (Epictetus, 3). That is, rather than ascribing special value to a single object, we should instead ascribe that value to its type—for instance, rather than having a particular favourite pet cat, we ought to like all cats. This is first of all a nod to universality, recognising that all things are equal (i.e., all cats are equally valuable as cats); but it is also a defence mechanism of sorts, to make the slings and arrows that necessity hurls at us easier to shrug off. When a favourite pet cat dies, it's understandable to be sad about it; but if a cat dies, it's easily replaced by another. (This also stems from the Stoic philosopher's rejection of the passions, or emotions.)
Thinking of all things universally in this way requires a certain degree of mental control, because everything is not the same as every other thing of its type. The Stoics admitted this freely; Cicero writes that "it is the nature of things that all things are unique and distinguishable one from the other, and that two or more objects never possess a common character differing in no way whatever" (SVF II, 114), and furthermore that "'no hair or grain of sand is in all respects the same as another hair or grain of sand" (SVF II, 113). But it is still necessary to overlook this uniqueness for the greater good of understanding (and applying) universality.
But at the same time, by this same principle, the individual takes on a heightened importance—every person becomes equal with every other person, regardless of their sex or social standing, because all are similarly driven by necessity. This is a radical departure from the social mores of Hellenic Greece, where individuals were nothing outside of the groups they belonged to, i.e. the polis, the family, etc. Perhaps the most telling example of this brand of individualism is that anyone could be a Stoic philosopher, even women, up to and including a king (Marcus Aurelius) and a slave (Epictetus). Thus even though the principles of Stoic philosophy are universal in their application, they serve to preserve each individual's worth.
Numeration of Stoic sources above is in accordance with the system used in von Arnim's Stoicorum veterum fragmenta, which is my favourite primary source for anything in the history of the world. Translations for these fragments are by Jason L. Saunders, from Greek and Roman Philosophy after Aristotle, published by The Free Press in 1994. The translations of Epictetus above are by P.E. Matheson, and are taken from the same volume.