Very interesting post, Mithadan. I've actually felt both griefs--the Harry one and the Tolkien one. The former has far more to do with nostalgia for a childhood suddenly past (I'm 19--I read the first Potter at 11) than a true sorrow over the end of the series (which isn't to say I didn't enjoy the books, because I did).
However, the Tolkien grief is just as vivid, if not more so, at least for me. I remember the day I finished LOTR for the first time--seeing the final page through a haze of tears, then drying my eyes and going back to the beginning. I read it five more times, because I couldn't accept that it was over, before reverting to the once-a-year structure that most Tolkien fans follow. I still, to this day, have not read the final chapter of the Silmarillion because when I arrived at that point, just a few pages from the end two years or more ago, it was the last "new" Tolkien of its kind on earth (I'd been reading nearly confusingly out of order--by then I'd finished Lost Tales, a fair chunk of HOME, and the UT, but all were far less complete). I didn't want to let go of the magic. I'm just waiting for a free Saturday to finish Sil and begin CoH...which I think I might just finish (
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It is true that the stories are still there, and true that they can be read again, and that wonderful new things can be discovered upon every re-reading...at least as far as Middle-earth is concerned. I'm not so sure about Harry Potter, since when I return to those books, I usually just pick out favorite moments and re-read those instead...a re-read of Lord of the Rings is much more time-consuming, because I re-read every word. I will always love Lord of the Rings...the one thing that I am still sad about is that it's never quite the same after you read it once.
I'm not sure what all this adds, so I guess I'll just stop now.