Loneliness is a signal, like hunger

Hunger is not a defect. It is a signal built to push you toward food. Thirst does the same job for water.

Loneliness works the same way. It is unpleasant on purpose, because for most of human history a person cut off from the group was a person in real danger. The discomfort exists to push you back toward people.

So far, so useful. The complication is what else the signal switches on while it is running.