Humans are social animals.
Long before humans had fully evolved, when they were still apes, they had already begun living in groups and learned things one person could never accomplish alone, such as dividing labor and cooperating.
After evolving into primitive humans, they learned to use tools. The methods of using those tools also spread gradually through group living, all because of an inadvertent human action.
Later, during the tribal period, humans divided into individual tribes, each of which survived and developed on its own.
Even today, humans still have no choice but to integrate into society. This was the unique sense of belonging that social animals felt when interacting with their own kind.
Feng Chen had known all this back when he was still studying high school subjects. Otherwise, how could anyone call Feng Chen an idiot?
Even after spending fifty thousand years with Feng Chen, Sirin was still a child in every respect. This included interacting with other people. Human age could roughly be divided into two types: actual age and social age—in other words, the length of time spent interacting with others and the experience gained from it.
As for the former, Sirin had been pushing thirty before she and Feng Chen returned to the past, so she was hardly young. But as for the latter, Sirin was clearly no different from an infant. Yet even an infant would feel uneasy when its parents were absent and then cry loudly. Only after hearing the crying would the parents come to the infant's side to comfort her.
A person's inability to integrate into society, an infant's unease, and so on were all caused by something humans inherently feared.
—Loneliness.
As mentioned earlier, humans were social animals, and this trait allowed them to develop rapidly with a population base