Some books should stand alone. Once the story has been told, to revisit the characters would rob the narrative of the depth of meaning. For such stories, sequels spoil them by diluting what made the first one great.
A good example of the dilution of impact can be found in the BBC television series, Doctor Who. The episode “Blink” introduced the threat of the Weeping Angels. “Blink” ends with the implication that these deadly hunters could be anywhere – so don’t blink; blink and you’re dead. (It makes sense if you have seen the episode).
blink and you’re dead.
With each return of the Weeping Angels, the writers had to present the angels in larger numbers but for less impact. This dilution effect continued until the writers had to make the Statue of Liberty (yes, really) an angel just to get a similar dramatic reaction. These sequels diluted the narrative impact of an otherwise remarkable Doctor Who story.
There are lots of good reasons to have a sequel. Almost all of them include having something fresh to say. If there is nothing to add save for retreading the same plot, your story would be better off without a sequel.
Ideally, any sequel you write will be as good or better than the stories that went before it. As good or better, does not mean the same as.
A good sequel is a story and of itself. Likewise, the book that births the sequel should be complete in and of itself. A book, comic, or movie should never just be an advert for the next book. DC, we are all looking at you.
Some stories do not need nor should they have sequels. Do you agree or am I wrong? Let me know your thoughts in the comments section.