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Tear Jerker: Lassie
The original Lassie. Any dog movie is going to make me burn through a box of Kleenex at least, but the the dealbreaker was a sequence in the middle of the film where a kindly old traveling tinsmith takes Lassie in for a while. The old man has a fluffy little dog that is his only companion. When thieves attack him, Lassie and the little dog rush to his defense and the little dog is brutally killed. And then that cold-hearted bitch Lassie LEAVES THE OLD MAN! Great, now my keyboard is damp.
Well, she had to—otherwise we wouldn't have had the even bigger TJ at the end when a depressed Joe comes out of school, and looks up to see a beat-up Lassie waiting under the same tree she has always been.