I like sequels. I love reading them; they're a return to familiar settings and characters with new twists. It reminds you why you loved the story in the first place (or the author, too). Sequels show you more of a world that you already were interested in, and series give the opportunity to tell many stories inside of an overarching main story, which can be used to glorious effect. (Keep all this in mind as you read on.)
I'm picky about sequels. Especially for myself. I plan to write stand alone fiction. (Okay, except for this one idea for a series, but that spans four different genres and plentifully diverse characters. So it doesn't count.) my reasoning for this is complicated, or at least I think it is. You see, first off, I don't get how authors write these seven to fifteen book series. My mind flits through hundreds of ideas, and no matter how many I mash together, there are still to many. If I focused on one series for that long, I would never get that many ideas out. It simply could not happen. Stand alone fiction gives me the opportunity to put down an idea and then check it off the list. Move to the next one.
A few more things. I feel like writing a sequel is a small bit lazy for authors. Sure, it's still a hell of a lot of work. Writing is hard. But you've already got a setting and characters and a whole back story finished and in hand. Yes, I give slack when new parts of the setting are explored, new characters introduced, and an interesting new story is presented. I still think that every novel deserves the attention and planning of the first.
Last point and then I'll stop (there are obviously more). Killing characters. It's powerful. You can move people when you kill characters. It's a sack of bricks to the chest when a character that you love dies. And (oftentimes) it's a huge driving point for the other (still living) characters. This is downplayed if there's a sequel. Sure, you can make passive references to the other character's death. Yes, you can make several passing remarks and even make it a lasting driving force for one or several character/s. But the death is still downplayed, because it already happened. It's done. And now you're missing that character, who no doubt (if you're making passing remarks to them) played a major role in the previous story.
I'll throw this out there again; I like sequels. I read them. However, I can't see myself writing them. (Save very few reasonable exceptions.) I'm not saying that everyone should just stop reading sequels, in fact keep reading them. I just think we should maybe be more skeptical about picking sequels as readers. And deciding to write them as writers. Not every story needs a sequel to be told.