Sequels can be good when they advance a story, (Star Wars, Back to the Future, Planet of the Apes...) BUT, often times sequels are a bad idea. A few examples of movies that didn't need a sequel or multiple sequels but had them anyway:
Police Academy (liked the first 3...didn't need 4 more!)
Revenge of the Nerds (no sequel required)
The Karate Kid (should have quit after the Okinawa trip)
Rocky (after the third, shouyld have skipped over 4 & 5 and gone straight to Rocky Balboa)
Rambo (2 was enough)
Now, movies that should NEVER see a sequel:
E.T., the Extra-Terrestrial
The Goonies
and ALMOST every Disney animated film. I liked the Aladdin series and The Little Mermaid triology, but some films need to be left alone. I remember seeing a featurette on my Dumbo DVD about Dumbo II and thought, "Why?"...but the film hasn't seen the light of day as of yet, thankfully. All in all, some sequels I feel work...and some don't.