I saw the other day that James Cameron has signed to do two more Avatar movies. I guess it was inevitable. Although I'm glad I'm not the person writing the story.
I can understand why writers want to do sequels.
As a writer, I invest a lot of time in world building. The world grows richer with each book I write. I have maps (waving to Chris), lineage trees so I can remember who is related to who and how old they were when things happened. I create different societies, with different customs and festivals. I end up with notes on time lines, lists of sayings and distances travelled by different means. All so that I won't slip up when my characters ride off on another adventure.
A writer friend once told me that she had a green room where her characters waited before they took part in another adventure. Unfortunately one of her minor character's arms grew back while he was in the green room. She'd forgotten he'd lost it in the previous book.
As a reader, I like going back to a world I know. It is feels as if I'm taking a holiday in a familiar place. Some of the features might have changed, new buildings, people may have moved on or gotten married, but enough is familiar for it feel right.
I loved going back to Lois McMaster Bujold's Barrayar books about Miles Vorksigan. Here you can see a list of events in Mile's life, which Lois has compiled.
Are you keen on sequels? Bearing in mind that fantasy books are published in trilogies, these sequels will often be another trilogy. Can you think of one stand alone book that you'd like the author to write a sequel to? (Don't worry if they author is dead, for the purpose of this exercise, we can reanimate writers LOL).