I don’t care what happens to these people.
The “eight deadly words” were probably not first uttered by writer Dorothy J. Heydt on Usenet in 1991, but she was the first to name them as such. They are the words that no writer or producer or actor wants to hear about their work. They are the words that readers utter when they put down a book, or viewers proclaim when they switch off a film or stop watching a television series.
Those words are “I don’t care what happens to these people”. If you don’t care, why should you read (or watch) on?
Now, “caring” about the characters is not necessarily the same as liking them. Humbert Humbert from Lolita is a vile excuse for a human being. If you find yourself liking him, please get help now. Eva Khatchadourian (We Need to talk About Kevin), J.R. Ewing (Dallas), Joffrey Baratheon (Game of Thrones): these are not nice people. However, you care about what happens to them. You might hope that really bad things happen to them, but you want to know what those bad things are.
“These people” might not even be people. In Watership Down, you come to care about rabbits; in Wall-E, you are emotionally invested in the fate of a robot.
Think of the fiction that has remained with you. The characters are real, and you want to know what happens to them. That’s the reason why readers clamored for more stories about Sherlock Holmes and Anne Shirley; that’s why viewers keep coming back for Tyrion Lannister and Walter White.
I get frustrated by clunky writing or unrealistic plotting or stunted dialogue. But the one thing that will make me stick with a story is that I want to find out what happens. The one thing that will make me put it away is when I just don’t care.