I think that my dislike of sequels can be traced back to The Empire Strikes Back.

In isolation, Star Wars had a triumphant ending. The kid from the boondocks rose to the occasion and became a hero.

He showed himself worthy in the eyes of the tough kid and the pretty girl and the wise old man, and he did something that would have made his father proud.

Empire took all that away from us. As the first film in a trilogy (or film number whatever in a series) the story of Luke Skywalker goes from being a boy becoming a hero to a naive recruit becoming just another soldier.

The final scene of Star Wars is a recognition of a hero who has saved the galaxy and won the admiration of the beautiful space princess.

The final scene of A New Hope is just a bureaucrat entering a commendation into a personnel file.

And that is why I don't like series fiction. In a series, there are no happy endings because in a series nothing ever ends.