I recently read an article about writing plot twists. It works in three steps: proposal, argument, and answer. What makes twists shocking is when they answer a question of the plot. The Strange Case of Doctor Jekyll and Mr. Hyde became a hit because of the twist. It turns out they were the same person. This answered the plot's mystery, as well as the theme of the story regarding a person's nature and dark side. Compare that to a story where two men are having a fight over a woman who yells that she's pregnant, without showing any signs beforehand, and isn't a part of the theme or plot.
The Strange Case of Doctor Jekyll and Mr. Hyde became a hit because of the twist. It turns out they were the same person. This answered the plot's mystery, as well as the theme of the story regarding a person's nature and dark side.
Compare that to a story where two men are having a fight over a woman who yells that she's pregnant, without showing any signs beforehand, and isn't a part of the theme or plot.