Seneca has a line on some of the core ideas of Stoicism:
Never give into adversity, never trust prosperity, and always treat fortune as if everything that will come to pass will.
Let's focus on the second idea: never trust prosperity.
The suspicion of pleasure and prosperous times may ring hollow if you feel like you're down in the dumps. But there are several reasons to be skeptical of when things are going well.
It can distort our judgment
Success brings fans and accolades. This can feel good but distorts our judgment. Consider intellectual accomplishment. Towards the end of his life, Freud changed his mind about one of his theories' details, introducing the death drive. By this time, he had become famous - the introduction of this idea met a lot of criticism, from people who had started to call themselves Freudians. The true fans! Whatever you think of the psychologist Freud and his work, he provides an amusing case of fans accusing the teacher that they don't understand their own theories. One wonders how many creators, from musicians to intellectuals, feared their audience so much that they allowed them to cut their potential short. Success can breed complacency. Because losing what we have may seem so dreadful, whether that's wealth, material comfort, or intellectual prestige.
Things fall apart
The natural tendency of things is to change and fall apart. While things are working, it can be easy to forget why they broke in the past. This often occurs during relationships. Dean Delis talks about partners getting back together after a split in The Passion Trap
But the joy of reuniting is no lasting cure for deep-seated relationship problems. This is in fact a good time for couples to enter therapy, because they're both motivated to make the relationship work. But it's also the least likely time, because their problems seem to have vanished. Unfortunately, it's usually only a matter of time before the partners fall back into their old behavior patterns.
Getting back together kicks off another honeymoon period. But the old behavioral patterns that brought about the split in the first place are still there, unaddressed. In the heat of the moment, it's tempting to imagine that they've disappeared. And sometimes they have, but often they haven't.
Just because prosperity can't be trusted doesn't mean it can't be enjoyed. The rush of success is like a rocket that can go careening into the ground or up into the stars. Other experiences are more tranquil. The peace may persist, or it may devolve into a stupor.
During times of prosperity, take time to question your assumptions, get harsh feedback, and improve your judgment. When things are working well, use the momentum to ensure that they're resilient and handle the stress of things going poorly.
Fix things when they're not broken.