Analogous: associative horizon is the low end of psychoticism. But we must not conflate a personality dimension with a passing emotional phenomenon (even if the emotion can last for years). Depression (or despair, in less modern parlance) is like ennui in that it is caused by a loss of belief in one’s own agency, but it differs in that a depressed person is consistently failing to realize their desires.
A person who thinks “money will make me happy” and puts forth their best effort to become rich, and succeeds and becomes rich and happy, he will retain the sense of agency. If the same person succeeds in becoming rich but being rich fails to make him happy, he will retain the sense of agency but suffer an existential crisis (as in ennui). If the same person puts forth his best effort and fails to become rich, he will become frustrated. A consistent pattern of frustrations, as perceived subconsciously, leads to depression.
There is obviously a personality dimension that predisposes some people to become frustrated more easily, and therefore depressed more easily. Compounding the problem in the modern day is a giant edifice of lies causing immense confusion about means and ends (Ex. “I’m nice to her, why doesn’t she like me?”), which explains a great deal of the rise in major depression diagnoses (and subsequent, misled pharmaceutical intervention). I’ll call this “agent sensitivity” for now because I’m running out of time.
“Agent sensitivity” can be changed somewhat by environmental factors, mostly by exposure to all four combinations of hard work-vs-laziness leading to success-vs-failure. If a person fails to experience one of these combinations until later in life, it will be a shocking experience that they won’t understand. However, the primary environmental factor in this trait is the memory of an entire lifetime of hard work or laziness leading to success or failure, leading to formation of pattern perception. For example, a person might realize “I worked hard at math and still failed, but I worked hard at piano and became successful.” This person would be sanguine about their potential to learn guitar, which is a lot like piano, but melancholy about their potential to learn physics, which is a lot like math.
In contrast, a person who has never experienced frustration- due perhaps to extreme IQ or extreme athleticism or simply by leading a highly sheltered life- is going to receive a hard shock when they are unable to achieve success in something.
To be absolutely clear about this, depression is a phase change from frustration, rather than merely being “a lot of frustration”. It is caused by “a lot of frustration” but they are not interchangeable. This is analogous to high trait psychoticism leading to psychosis.