So much of the hard work for an author is creating the rules of your own world. Who are the characters, what is the weather, who is related, what form is the magic, what has changed in this world, who is the outsider, where do people go to meet--the list is practically endless. And once an author has created a world--whether contemporary or fantasy--it's easy to just continue with that setting.
But the author then has to figure out a couple of things. Is it a tight series with a definite end? A mystery to be solved over four books, three siblings to get married off or such? Is it a loose series, where it just continues with a focus on different characters in the town? (just be aware you might be writing to diminishing returns if it goes on too long) Are you going to be able to continue to publish this series with some regularity?
And of course an early decision is will the series focus on one character or couple over the course of three to a dozen titles? Or will it move between different viewpoint characters with a romance happening in each book?
Although sometimes you might just fall into a series, I do think that once you realize you have more characters and conflicts to explore, you need to start laying out the groundwork and giving some thought to your overall goal and direction. You've got to start with compelling characters--and also some fascinating secondary characters that will have an impact.
And you've got to allow some opportunities for growth and change over the course of the titles. Sometimes you also have to keep in mind what is happening in the character's lives--only six months have passed in three books!--but also what's happening in the reader's attitude--six years have passed and nothing has happened!
One other thing to keep in mind in later books--while it's great to revisit the first couples, if each book has the ten previous couples making an appearance it might be taking up too much of this book's focus. You don't want to spend time explaining who every other person and their relationship is. (OTOH, I do recommend a section on a website listing the central characters of each book so a reader can quickly find the one they need!)
There is so much more to consider with a series I'll no doubt get around to it again.
Meanwhile, any favorite elements in a series?