Sometimes of course authors write a story with the intention of continuing it. Sometimes that happens and sometimes it doesn't. I've been encouraged to write sequels to stories a number of times and I guess I've done so only in about a quarter of the cases. On the whole I find the sequels are less subtle than the starters and there is a problem with developing the characters so that they don't become like cartoon characters, never changing, but they don't develop in ways which undermine the appeal of the original story. If story 1 ended with snooty Jane and lowbrow Jack finding a common interest and getting together, that's a happy ending that will please many people. So how do you maintain interest in the sequel? Not impossible, but quite hard.
Some stories, though, quite lend themselves to sequels, if they're not already part of a series. If the story of the policewoman's search for the phantom panty collector ends with her key witness demanding to spank her before revealing the identity of the criminal, spanking her and then vanishing with her panties, it asks for a sequel in which she pursues him. | 
