Nowadays it seems like most books end up becoming part of a series. Even books that started off as a trilogy at times end up becoming a series of more than three novels. The Hunger Games trilogy written by Stephanie Collins however seems like it will stop at Mockingjay given the ending.
YA novels are more popular than even and I think it’s the film industry along with lucrative book deals for series that have driven writers to continue writing after the first book. I think it’s a great thing. So far all the series I’ve read I’ve enjoyed. Writing complex story lines helps writers to be able to keep on writing compelling novels. Adding interesting subplots is another way to keep the series going like in the novel I am currently reading Unsouled by Neal Shusterman. Shusterman set up the novel by introducing two key characters whose life readers followed. At times Shusterman even adds more subplots allowing readers to be introduced to new storylines.
The Life as We Knew It series written by Susan Beth Pfeffer also had several subplots which actually allowed reader to read books about different characters. As long as the characters are compelling having multiple subplots seems to work. So if you’re planning to create a series it would be a good idea to map out the story and give some through to how you plan to lay out the series before or as you write the very first book.