Speaking for myself I have a rotating circle of friends, some of whom still play, many of whom are semi- or probably permanently retired. I've occasionally been tagged by some of those to get back in the game but usually it's when I have the itch.
I've noticed over time the cause and effect circle @Wizz mentioned - reading geeky fantasy books which led to playing similar things out on a MUSH - had even been reversed; I'd read new novels while playing the possibility of turning it into a game at the back of my head. What would The Stormlight Archive or the Lightbringer setting look like, would it be playable? Could it lead to interesting gameplay if the plot was detached from the book characters?
The reality of it sometimes disappointed me. Perhaps it's because of what I do here and the stories I hear both publicly or - even more frequently - in private, but I've found the less I need to care about the OOC side of MU*ing the more engaged I allow myself to become, and the happier I am on a day to day basis with it.
But what draws me the most is moments when things click.
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That feeling days after I start a new character when the other shoe drops and the PC does things on his own I didn't know he would when I started typing his next pose; it's magic I haven't found anywhere else.
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The time when I meet another PC whose player I don't even know and we harmonize over theme; it just flows, one pose into the other, tossing things over only to see them caught and returned, enriched in each bout.
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The notion that a character is a living entity in a way who can outgrow my initial plans, changing from what he experiences which are in themselves things no one planned for them; not staff, not other players, no one. They just happened, and he's now a different beast than before.
I haven't ran into the opportunity to get those feelings from other types of gaming, and although I wouldn't know, it's probably not quite the same for creative "solo" writing either. But if I do get the itch to get back it's because I'm missing them.