In 2011, scholar and cultural theorist Lauren Berlant published a book called Cruel Optimism, in which she outlined a concept—oriented around an idea called "affect theory"—that essentially means we're prone to a double-bind in our desires, shaped by fantasies that serve as their own hurdles.
Said another way: many of the things we aspire to do, to experience, to feel or think or become, are themselves the things (or come tandem with secondary elements) that keep us from doing, experiencing, feeling or thinking or becoming those things.
In this context, "affect" refers to our subjective experience of something.
Someone with a "positive affect" will be generally optimistic and happy-ish, probably responding to whatever life throws at them in an upbeat way.
Someone with a "negative affect" will do the opposite, perceiving anything that happens to them or the world through a generally downbeat, pessimistic lens.
Affect theory serves as an organizational method for affect across different fields, ranging from medicine to literary theory to gender studies, so it means somewhat or radically different things to different people, depending on what larger field of inquiry they're operating within.
In Berlant's estimation, this theory informs our optimism by nudging us toward affective experiences, rather than practical ones.
We yearn to be successful according to cultural ideals (the American Dream, for instance), but the tenets of that Dream are often self-defeating, causing us to expend vast resources (monetary, energetic, chronological) on things that don't get us any closer to the implied stability, safety, and happiness we assume we will experience once we achieve this goal.
Our attachment to this goal—according to this framing of the issue, anyway—might thus be the very thing that prevents us from ever attaining it.
She's labeled this type of optimism "cruel" because there's a sort of perverse horribleness in being tormented by something that can simultaneously help us get up in the morning and allow us to wade through sadness and defeat and pain toward a beacon of inspirational light on the horizon.
There's an element of tragedy in this dynamic, too, in that we're often incapable of seeing our desires for what they are and stepping away from them, because they're often fundamental elements of our worldviews and self-perceptions.
This concept has been further elucidated and spun-off by other thinkers in the years since Berlant's book was published, often used to gesture at how the nature of our economic system can incentivize the production of goal-sustaining excuses: taking yoga and meditation classes to deal with the stress that results from the nature of our jobs, which we work as part of a larger effort to achieve the wealth and prestige associated with a specific type of lifestyle we crave because we believe it will bring us peace, for instance.
It's possible, according to some thinkers who have tackled this topic at least, to swap in authentic optimism for the cruel kind if we're conscious and careful about it, and allow ourselves to identify, investigate, and directly address the causes of these issues, rather than looking past them as we otherwise tend to do.
But it's also worth having this label and framing on hand to help us organize and better perceive what can otherwise be tangled, blurry, and tough-to-discuss concepts; even if this is just one way of partitioning and parsing such things.