Old Yeller is really sort of the gold standard of destroying the innocence of children everywhere. It's funny, most of us can watch humans getting mercilessly hacked to bits without blinking an eye, but as soon as you start to bring animals into the occasion, that's crossing the line in a major way. We have far more empathy for anonymous cats and dogs than we do for anonymous humans (at least in the movies). On some level, maybe it's because we know that the humans are probably jerks anyway, whereas animals by their very nature are innocent. So when you watch a movie like Old Yeller, which features perhaps the most famous incident of a beloved dog having to be put to sleep, it's like someone rips your heart out of your chest, stops on it, and lights it on fire. Everyone who has ever had a pet can empathize with the tremendously difficult decision to euthanize them, as young Travis is forced to shoot his faithful Mastador dog who was infected with rabies while defending the family from a wolf attack.