This is a toy psychological theory which tries to explain why gratitude practices and stoicism have good effects.
Your "need-set" is all the things you need. More specifically, your need-set is the collection of things that have to seem true for you to feel either OK or better.
Broadly, we have two kinds of motivation: positive and negative. Positive motivation is when you want something and try to move towards it. Negative motivation is when you want to avoid something and try to move away from it. Positive motivation is much more detailed in its targeting and tries to do as well as it can. Negative motivation is often faster and scrambles to find any acceptable solution to the problem. From the inside, positive motivation often feels relaxed or enthusiastic, while negative motivation often feels frozen or frantic.
When you have everything in your need-set, you generally experience positive motivation. Here, you pursue things that are not in the need-set that are just nice-to-have. Conversely, when something in your need-set is missing, you generally experience negative motivation. When something you really need is missing, you may not care so much how you solve the problem, just that it goes away.
The need-set expands and contracts adaptively. When you have something good for a long time, and it seems very reliably there it often gets added to the need-set. This is called "taking things for granted". When you lose something in your need-set and finally, after attempts to get it back, give up, the thing drops out of the need-set. This process often gets called "grieving". Of course, this is oversimplified; these are not the only processes whereby things enter and exit the need-set.
In the modern world, negative motivations are overused. This is partially because the environment we evolved in is far harsher than the one we find ourselves in today. As such, negative motivations seem, to a significant extent, selected for running away from lions, avoiding getting killed by the other tribe members, and the like. However, in the industrialized world, we have basically no remaining natural predators (the ones we see tend to be locked in cages for our amusement) and violence is at very low levels. As such, because of our (comparatively) vast wealth and cushy lifestyles, we have many opportunities to take things for granted, thereby creating opportunities for negative motivation.
A subtler psychological peril is that pleasant false beliefs can linger and then enter the need-set. The resultant sort of negative motivations to protect false beliefs seem to be the source of a fair number of cognitive blind spots and egoic defense mechanisms.
For these reasons, greater psychological health can be obtained by shrinking the need-set or by preventing it from growing unnecessarily, because shrinking it would result in more states of the world invoking positive motivation rather than negative motivation. Stoicism, with its negative visualizations, wards off taking things for granted; even a visualization counts towards not taking something for granted.
Gratitude practices may do a subtler version of this. While the visualizations may be pleasant, saying one is grateful for something seems like it may (at least subconsciously) involve comparing it to a world in which that thing does not exist. Additionally, by cultivating good feelings about the things one already has, it may aid with the grieving process; the process is likely to be set up to struggle less when letting go of a supposed need, the more positive feelings one experiences. That said, there are also non-need-set related positive effects gratitude practices have. For example, I think they also put one more in touch with one's positive preferences.
Stability's value is as a loss-prevention or expense-prevention resource: a status of being predictable or being resistant to immediate entropy in some way. It's such a broadly applicable concept that its benefits are practically ubiquitous, and it adds all the types of value to various circumstances.
Stability of a situation, as in the expectation of not having to anticipate much change, allows you to conserve resources you might otherwise need to devote toward anticipation of contingencies; you can also thus experience the opposite of anxiety.
A medical patient who is stable is in less danger of dying; this kind of stability is a resource both to the patient's continued existence (agency, experience, utility to society as a resource) and to their medical team who don't need to expend resources to immediately and actively maintain the patient's life medically.
Being seen as a stable person by the standards of a given group grants you esteem from that group, because you're predictable and will not cost them sudden, unexpected loss of things of value such as their group's esteem in the eyes of whichever society they esteem. A person being perceived by police as mentally stable and/or morally stable (in the sense of being unlikely to commit assault or other crimes) grants the police a sense that you're predictable and thus not an immediate danger that needs to be violently subdued. In other words, you have the esteem due a member of the law-abiding community.
Stability of government gains a country more opportunities for international trade (resources) and gains its citizenry and businesses a credit rating (which is a resource based on how much one is esteemed as a reliable payer of debts by lenders).
Stable isotopes, which are not radioactive, are radiologically safe to touch or handle. However, this doesn't mean it's entirely safe! Lead is not safe to touch because even though it's stable enough to be used for radioactive shielding, it has neurotoxic chemical effects. For low-energy purposes such as home-building, stable chemicals and elements are more valuable resources; for high-energy purposes, such as weapons or manufacturing, unstable chemicals or elements are more valuable resources.
That was a lovely example, thank you!