But, what made me turn it into a series was that I didn't want to leave that "place" yet. I wasn't ready to say good-bye to the characters, or to the setting (the beautiful Cotswolds). I still had stories to tell. And so, the series idea was born.
I'm sort of glad it came about that way -- that I didn't really sit down and plan out a series from the get-go (nothing wrong with that, of course). I love that it just seemed to make sense, that when I neared the end of Book 1, I wasn't ready to leave. It showed me I was on the right track, expanding into a series. And that, just maybe, if future readers picked up Book 1, they too would want to stay a little longer (and, thus, buy Book 2, lol).
I'm no expert on series, but I'd say that if you're thinking about doing one, make sure it's something you can sustain over more than one book. And that you, as the writer, won't bore easily of the setting/characters. Because when you get bored, so will the reader.
Another "plus" for the series (besides the obvious marketing advantage of people having the incentive to read more than just one book) is that, as a writer, my setting is already fixed. Most of my characters are already in place, already fleshed out. It's not that I don't still have work to do, but much of the time-consuming parts of writing a first novel take care of themselves in the second, third, fourth books of a series. Thus, I find that I have more time to devote to plot, since I already know the setting and characters so well.
I never, ever saw myself as a "series" writer. Never. It wasn't a goal of mine. But, somehow, I guess I have become a series writer. And I must say, I love it!