When I was a little kid, I had no concept of loneliness other than being terribly afraid of being alone too long as is natural for little children who are left alone at home all day.
Now, I get sort-of lonely for practical progress and change that has to involve one of my major interests or my personal situation. I usually feel just as alone when I'm with someone else as I would be if I were completely alone. Most of the people in my life make me feel claustrophobic when they are in the same house, so when they're gone, I feel this amazing peaceful feeling, often even when I am not aware that they have left.
I call it being lonely for intelligent conversation. I don't really get lonely for people, and I certainly don't miss people once they're gone, no matter for how long. I do miss intelligent conversation though, and I do sometimes miss practical progress and positive developments in my life.
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Still alive...