Humans are uniquely social creatures. There are other animals that tend to congregate in packs or hives or whatever due to evolutionary pressures, but I don't think they experience loneliness in the ways humans do.

Just look at what happens in solitary confinement. It literally drives inmates mad (and yes there is a bit of a confounding variable of this taking place in a prison, but the general principle still applies).

We have the phrase "No man is an island" for a reason. Although I'm sure you can always find exceptions, humans generally do not well without other humans. We have evolved to be in groups. The need for others is not quite as fundamental as the need for food and general physical safety, but it is not more than a step away. This is who we are as humans.

The modern world doesn't seem well equipped to address this need. We've lost most of our 3rd places, the youth in most developed countries face crushing economic circumstances, and our world feels more disconnected than ever.