Some people love to read series books. Others hate them. And a few--eh, they can take them or leave them. Writers are much the same. Some writers make a deliberate decision to write stand alone books because they revel in developing new characters, settings, and conflict. Others become enamored of their cast and enjoy getting to knew them better from book to book.
I, myself, follow both lines. I enjoy the details of setting up a new book--the characters, setting, the world building--and I also enjoy exploring the peripheral characters in succeeding books. There are advantages and disadvantages to series writing. There are readers--like me--who are relatively rigid about reading series books in order, even if they are, in essence, stand alone books. If the number of books in the series grows too large, new readers who may discover the series after a number of books are released feel like there are too many to purchase in order to "catch up." That is a valid concern. I have two series currently with several books in each. The books in both series could be read as stand alone stories. I worked very hard to make that so. But I freely admit that the readers enjoyment of the stories will be enhanced if they read them in order.
On the other hand, the advantage to a series is the immediate familiarity with the cast of characters. For a reader who has read Nora Roberts' In Death series, the varied interesting cast is comforting and familiar. Because the reader knows the characters' backstory, there are some things that don't have to be explained. For the reader who has limited time to read, series offer them the enjoyment of reading, while allowing for a certain shortcut in figuring out characters' motivations. I think that might be the attraction of reading a series.
Now writing them is an entirely different horse of another color. While it might initially be more work to invent a new world with characters and background, it is a lot of work to maintain that world through book after book. The devil really is in the details. Lots and lots of details that seem to increase exponentially with each new book. Tons of details that depend on how elaborate the world building was to begin with. My series do not take place in the contemporary world so even the smallest details must be cataloged... plants, animals, buildings, laws.
My current work in progress is a stand alone time travel. There are days when it seems to me that there are more details for this one story than all of my series books together. Yet, I know that isn't true. Already, one beta reader has commented, this will be a great series. Excuse me? No. Stand alone. One book with a beginning, a middle, an end. Finished. Because unlike a stand alone, series never seem to be finished. There's always one more character that beckons. One character that begs you to tell his or her story.
Series can be never ending.