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Tear Jerker: The Baby-Sitters Club
The death of Louie; Mimi's stroke, deteriorating health and eventual death; and a lot of Abby's autobiography.
Louie's death is particularly bad because he doesn't simply die of old age. He had to be put down. Readers went through several books seeing Louie the collie as an energetic, happy, beloved family pet and see him slowly become weak and sick to the point where the Watson-Brewers admitted that they basically had to put him to sleep, because they couldn't bear for Louie to be in such pain. There's tons of little moments throughout, too, like David Michael trying to keep a half-blind Louie from going down stairs and getting hit in the eye for his trouble, David Michael opting to sleep downstairs with Louie during his last night home, the funeral...
Jessi's baby brother, Squirt, getting hurt in a car wreck.
Stacey having to choose between living with her mother or father. She knows one of them will be hurt either way.
There's something about the series' concluding that hits hard: as cheesy, unrealistic and downright irritating as the books could be at times, waving goodbye to these characters feels like leaving old friends behind. Or maybe that's just the nostalgia value.