It's a short prose written exactly in one of those moments of divagation these sorts of anxious thoughts linger on. That means it not only talks about the fear of loneliness but was created at a moment the author was feeling troubled by those very fears in an almost metalinguistic kind of way.
Ever since I can remember, I've always had a very exquisite fear: that of not being my best friend's best friend. That is, on the rare occasions I had a "best friend" as per se. Maybe that can be translated somehow as part of a much more primeval fear: that of being alone. It may sound incongruous, as I really love being alone - that's how I've lived my whole life and I don't intend to change. That said, people never "intend to change", they just do. But there's alone and there's Alone, or you could say that it's okay being by yourself most of the days, I even like it. On some special days, though, on a full moon or something like that, I don't want to be alone and I can't be alone and I simply need someone to unconsciously affirm me: "you are not alone". And being by myself on days like those is the worst thing I have ever felt.