Intellectual as an adjective, relating to the intellect. Without any claim to degree of intellect. So as opposed to emotional, physical, or spiritual loneliness. This is a loneliness of the mind, intellect.
I see it as a product of specialization. So many of us put so many hours of our life into things that few other people can understand or relate to. It's not that these other people aren't smart enough to understand what we do. It's simply a matter of hours of attention dedicated to a specific topic. Think of it like branches on a tree. The tree of collective human knowledge keeps growing and consequently, individual knowledge becomes more disparate.
