When I wrote the review for "The Lonely City by Olivia Laing", I found myself going back and thinking about the following statement taken from the first paragraph of the book.
"You can see them, but you can’t reach them, and so this commonplace urban phenomenon, available in any city of the world on any night, conveys to even the most social a tremor of loneliness, its uneasy combination of separation and exposure."
This thought is presented as fact, but it also is how the author sees loneliness. If you are socially minded you will be quicker to see your own isolation.
Some like solitude, and it may take a lot more than not being able to reach all the people you see, to cause loneliness. Perhaps for them it is in not being able to find their purpose?
Loneliness may be common, but the causes, and how it feels, are not common in how you explain it.