Howard Jacobson has written a piece on sequels in The Guardian; it’s about TV shows but speaks to novels too.
He argues that due to the nature of TV funding, anyone who writes a series never knows whether it will get commissioned for a second season. In the hope it might happen, the scriptwriters tend to leave unanswered questions, near-dead people alive and near-alive people dead in order to create the twists and tics for the hoped-for second season. And this is detrimental to the narrative of the first set of shows.
Indy authors are fond of a series too. We do this because it is hard enough finding someone to read your work. If they like it, you want to feed them with more stuff they like and a story involving the same characters is the easiest way to do this. I have The Heist and The Getaway, for instance, as the first tow installments in the Lagotti Family series.
Does this mean I left matters open? You bet I did. Was this to the detriment of the story? That I’m less sure about. You see, my first draft was so open it annoyed the initial readers. My original conception was to not cover any of the actual Heist. To stop the story just as Frank enters the bank, but people wanted more closure than that. So I feel as though I’ve got a more shut down piece than I thought I wanted – but now readers are more likely not to want to perform violent acts upon my person. A win-win for all concerned.