For a Stoic, the summum bonum is a flourishing life or eudaimonia. "eu" means 'good' or 'well' and "daimon" is 'soul' or 'spirit' (see this link). According to Stoic physics, each human has a bit of divinity within him and for the human to fully flourish, he must live in accordance with God or Nature.
Pierre Hadot cites Chrysippus in The Inner Citadel,
The definition of the happy life, according to Chrysippus, is that in which everything is done "in accordance with the harmony between the daimon within each one of us and the will of the governor of the universe" (p. 123).
The translation of this can also be found at The Project Gutenberg EBook of The Lives and Opinions of Eminent Philosophers, by Diogenes Laërtius - see Book VII, Zeno), in which it reads,
Again, this very thing is the virtue of the happy man and the perfect happiness of life when everything is done according to a harmony with the genius of each individual with reference to the will of the universal governor and manager of all things.
This is the basis for the Stoic phrase, 'live according to Nature' or 'live in agreement with Nature.'
Furthermore, the Stoic philosophy maintains that achieving eudaimonia is something which is 'up to us.' We are not dependent on externals in order to achieve this. Hence Seneca writes in this letter,
joy which springs wholly from oneself is leal and sound; it increases and attends us to the last; while all other things which provoke the admiration of the crowd are but temporary Goods.
This well-spring of joy or eudaimonia can be found regardless of goods or ills (externals) in our life. In fact, the externals or indifferents are the material for our use, in order to demonstrate arete which demonstration leads to eudaimonia.
For men make a mistake, my dear Lucilius, if they hold that anything good, or evil either, is bestowed upon us by Fortune; it is simply the raw material of Goods and Ills that she gives to us – the sources of things which, in our keeping, will develop into good or ill. For the soul is more powerful than any sort of Fortune; by its own agency it guides its affairs in either direction, and of its own power it can produce a happy life, or a wretched one (emphasis added).
At a fairly recent local Stoic meetup, we were talking about how Marcus Aurelius viewed wealth. I forget which passage it was, but the idea conveyed by Marcus was that riches and wealth are indifferents. One of the attendees wondered if what Marcus was writing about wealth was simply propaganda - basically telling people wealth didn't matter (so why should you pursue it). I replied that, first, it could not really be propaganda because Meditations was meant for himself and not for others' consumption; and second, that the Stoic view of wealth is simply that of material for good use. The goal is not to pursue wealth, but the goal is to demonstrate wisdom - wise use - of wealth. The same can be said of poverty. Seneca confirms this view when he notes that wealth or poverty are simply 'raw material' for a person to use in order to demonstrate what is uniquely 'up to them.'
Seneca further elaborates,
upright and honest man corrects the wrongs of Fortune, and softens hardship and bitterness because he knows how to endure them; he likewise accepts prosperity with appreciation and moderation, and stands up against trouble with steadiness and courage.
One of the Stoic exercises, which both help him prepare for and endure fortunate and unfortunate events is premeditatio malorum. I've often wondered if the 'malorum' should be dropped from this phrase. A Stoic who is comprehensive in his premeditation practice would consider all types of events - ones preferred and dispreferred. Seneca writes of this practice,
If you are thus poised, nothing will affect you and a man will be thus poised if he reflects on the possible ups and downs in human affairs before he feels their force. ... Be sure to foresee whatever can be foreseen by planning. Observe and avoid, long before it happens, anything that is likely to do you harm. To effect this your best assistance will be a spirit of confidence and a mind strongly resolved to endure all things.
And perhaps most of all, the premeditation we should always consider is the loss of life.
men are so wayward, and so forgetful of their goal and of the point toward which every day jostles them, that they are surprised at losing anything, although some day they are bound to lose everything. ... We must lose our lives as surely as we lose our property, and this, if we understand the truth, is itself a consolation. Lose it with equanimity; for you must lose your life also.
Other scenarios we ought to reflect on are fire, crucifixion, poison and exile.
The last part of the letter has an interesting tid-bit. We know that Stoics don't shy away from suicide. If one's death or suicide is a noble act, then he ought to carry it out. But when should one carry on living versus dying?
In continuing to live, he deals generously. Some other person might have put an end to these sufferings; but our friend considers it no less base to flee from death than to flee towards death ... if he can no longer be of service to anyone [then he should consider dying].
The man suffering from pain must make the wise choice.
he trust himself in the face of both; he does not suffer with resignation because he hopes for death, nor does he die gladly because he is tired of suffering. Pain he endures, death he awaits.