Before I delve into how I perceive things work we need to state some things about how youkai actually exist in the world. Essentially, youkai are beings who are different from human beings.
While humans exist through their physiology and biology, youkai are incorporeal beings who rely on the perception or interaction with humans such as by harming them in order to exist. As a result, if humans disappear, the existence of
youkai would also be in trouble.
In addition to all of that information, youkai are beings ruled by their spirit, their emotions and feelings and if they try to ride themselves of their desires, they will deny them selves of their existence. This is the most important aspect of their existence as my interpretation hinges on this fact.
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So I'll now go ahead and explain my interpretation of this sort of unconventional perception youkai possess. Here is the whole paragraph:
This is probably just nostalgia coloring my recollection. Time flows much more slowly for us youkai than it does for humans. Unsurprisingly we have much fonder memories of times past. This is probably because both humans and youkai, indeed all living beings, live out their lives in a series of harsh experiences. Living creatures who can't do this become controlled by a sense of resignation since everything that has happened in the past was so much worse than the present, so they probably tend to see everything as bad. Being able to look fondly on the past is a necessity for the long of life; those who are unable to do this and always think that what's happened in the past was bad essentially have no future to look forward to.
"This is probably just nostalgia coloring my recollection. Time flows much more slowly for us youkai than it does for humans." — This part is self-explanatory, time is perceived slower for youkai than it does for humans which results in youkai gaining longevity as a result of their unconventional perception of time. This is also the titular aspect of debate with people disagreeing what this entails and for how it is possible, so I'll see if my interpretation can help explain this aspect.
"Unsurprisingly we have much fonder memories of times past. This is probably because both humans and youkai, indeed all living beings, live out their lives in a series of harsh experiences." — This statement brings up the experiences of humans and youkai, not surprising considering their history with one another, but it also concerns all life which should include Vengeful Spirits, Divine Spirits, Hourai Immortals, etcetera. But to make things easier I'll simply focus on youkai and humans. These two species live out their lives in a series of experiences of either positivity or negativity. Humans with their shorter lives have less fond memories of the past than youkai do for the simple reason of their lifespan as well as physiology. They are biological, material, beings whose bodies grow into adulthood only to decay and weaken to death. Youkai are immaterial beings whose existence doesn't hinge on such things as their life cycle is consequentially different from human beings entirely. This will work in conjunction with the next part.
"Living creatures who can't do this become controlled by a sense of resignation since everything that has happened in the past was so much worse than the present, so they probably tend to see everything as bad." — Living beings cannot live as long as youkai do, as a result their harsher experiences build up into a sort of negativity of which gives them a sense of
resignation due to their higher tendency to view everything poorly. The term "resignation" is interesting as it can be synonymous with the term "surrender" or the notion of "giving up." It is almost as if they let their negative feelings and emotions rule over them they will resign from living. Of course, this does not exactly apply to human because of what I said, but it certainly does for youkai as I'll explain next.
"Being able to look fondly on the past is a necessity for the long of life..." — Youkai are able to look fondly to the past of life because of them perceiving time more slowly, but at the same time as Saikou himself as well as this part brought up:
It doesn't matter in this context. You can clearly see in the conclusion of the paragraph that the point is "Those who live long HAVE to see the past fondly, otherwise they get overwhelmed by the negative experiences they accumulate". It very explicitly spells out what the previous statement means. And so reading it as "Time flows slowly for them (metaphor for them being long-lived and thus living life more slowly than short-lived people) results in them seeing the past more fondly, as long-lived beings who wouldn't do that would be overwhelmed by bad experiences"
Youkai also have a sort of need or obligation of doing so as the build up of negative feelings and emotions would cause feelings of "resignation" and as I said earlier, youkai are beings ruled by their emotions, so feelings of resignation could potentially lead to their very existence being at risk, or some other results that are not good for them. The bottom line is, feelings of negativity is unhealthy for youkai. On the other hand, the ability of youkai to view the past as well as their experiences in a positive light allows them to suppress their negative emotions and live on even longer. This ability is a necessity. In fact, this paragraph implies, if not explicitly states, that the only way this sort of thing is possible is for youkai to perceive time itself differently from everyone else.
"...those who are unable to do this and always think that what's happened in the past was bad essentially have no future to look forward to." — This is the part that, in a way, reinforces what I explained. People who are unable to view their lives in a positive manner would essentially have no future to look up to, this doubly applies to youkai who are ruled entirely by their feelings and emotions and, as a result, would "lack a future to lookup to" so to speak. Whether this would mean that their existences in themselves would be at risk is something that is still not clear to me, but I do seek some elaboration or additional explanation from either Saikou or Mokou.
Speaking of Saikou, I wonder how and why he came to the conclusion that this statement: "Time flows much more slowly for us youkai than it does for humans." is somehow metaphorical and not meant to be taken literally. So I would like some elaboration on that point specifically. I know that what I explained teeters onto headcannon territory, but I feel that something similar can be said for Saikou's and Mokou's points of view. I just hope that my interpretation at least makes things a little bit clearer to people in this thread.