Success brings an asymmetry: you now have a lot more to lose than to gain.
You are hence fragile.
When you become rich, the pain of losing your fortune exceeds the emotional gain of getting additional wealth, so you start living under continuous emotional threat.
Seneca fathomed that possessions make us worry about downside, thus acting as a punishment as we depend on them. All downside, no upside. Even more: dependence on circumstances—rather, the emotions that arise from circumstances—induces a form of slavery.
Seneca’s practical method to counter such fragility was to go through mental exercises to write off possessions, so when losses occurred he would not feel the sting—a way to wrest one’s freedom from circumstances. It is similar to buying an insurance contract against losses. For instance, Seneca often started his journeys with almost the same belongings he would have if he were shipwrecked, which included a blanket to sleep on the ground, as inns were sparse at the time (though I need to qualify, to set things in the context of the day, that he had accompanying him “only one or two slaves”).
I would go through the mental exercise of assuming every morning that the worst possible thing had actually happened—the rest of the day would be a bonus.
An intelligent life is all about such emotional positioning to eliminate the sting of harm, which as we saw is done by mentally writing off belongings so one does not feel any pain from losses. The volatility of the world no longer affects you negatively.
My idea of the modern Stoic sage is someone who transforms fear into prudence, pain into information, mistakes into initiation, and desire into undertaking.
Seneca proposes a complete training program to handle life and use emotions properly—thanks to small but effective tricks. One trick, for instance, that a Roman Stoic would use to separate anger from rightful action and avoid committing harm he would regret later would be to wait at least a day before beating up a servant who committed a violation.
Seneca also provides us a catalogue of social deeds: invest in good actions. Things can be taken away from us—not good deeds and acts of virtue.
He said that wealth is the slave of the wise man and master of the fool.
Seneca even outlined his strategy in De beneficiis, explicitly calling it a cost-benefit analysis by using the word “bookkeeping”: “The bookkeeping of benefits is simple: it is all expenditure; if any one returns it, that is clear gain (my emphasis); if he does not return it, it is not lost, I gave it for the sake of giving.”