Update: After I finished writing, I don't seem to have gotten too far. This probably isn't of much interest, but I haven't said anything I don't think I can make public. So:
When I come home from a good con, or a good walk in the woods, or a good romp with baby tigers, and realize that I have to go back to my normal day to day life, I do often get depressed. It makes me realize that my life is made up of long stretches of merely existing, marking time, waiting for something really worthwhile; the only time I feel happy during those stretches is when I can escape into an imaginary world, either in a book or in my own mind. I'm not usually consciously unhappy, but I'm just kind of listlessly coasting through my days. And the longer I go between those times when I get to rise above just existing and actually enjoy life, the harder it is to remember that there should be more to life than just existing. And the harder it gets to remember, the harder it is to overcome the inertia to actually get up and do something when I could.
As I alluded to above, I have some activities that give me joy to make my life meaningful that aren't fundamentally about interacting with other people. Experiencing nature, whether it's taking a walk near my house in the country or driving to somewhere where there's mountains or water, is basically solitary; and interacting with animals is between me and the animals. And I do have some social interactions with people that I play games with or at Irish music sessions when I can get to them. But with only a couple of exceptions (and those are people I never see anymore), all of the people I can really call friends I know from fandom, and almost all of those people are filkers, and I only get to see them a few weekends a year. This leaves me pretty deprived socially.
Although I can function well enough in social situations and interact with strangers politely, without friction or obvious unpleasantness, I've never formed close connections easily. I can come up with several reasons for this. I don't process non-verbal cues from facial expression, posture, etc., properly; I think I may have a mild form of autism. Unless I really make a tremendous conscious effort to watch the person I'm talking to, my gaze wanders all over the place, which means that in addition to not picking up on important things the people around me are sending, I am inadvertently sending a message that I'm not really interested or paying attention. When a conversation or other social interaction is ongoing, I tend to linger on the sidelines, because by the time I work out what I want to say and manage to convince myself that the people I'm with want to hear my opinion, someone else has started talking. I have a very strong sense of other people's privacy, which keeps me from wanting to intrude. And finally, I seem to have a personality that focuses too much on the worst case scenario, so when I'm thinking of adding a comment to a conversation or reaching out socially to someone, I fixate on how I might offend or annoy, and let the chance slip past.
These difficulties in forming connections affect me even in fandom, but when I'm with other highly intelligent people who are clearly showing interest in something I both know and care about, it's much easier for me to get over my barriers.
I started writing this with a feeling that I was heading in the direction of some meaningful conclusion, but it seems to have eluded me. I'm going to post it anyway; if you read down to here, I hope you don't feel like I wasted your time.