Why is it that any damn kids' book with a dog in it always ends up with "very, very sad times" at the end? Whose idea was it to kill off all those damn dogs? It never fails! Show me a book with large type and a happy damn dog on the front, and blammo, I guarantee the poor dumb animal's days are numbered, as sure as I stand here.
This is a special case of the Disney Principle, which applies to all cute animals: The inexorable and bloody-handed logic of children's entertainment dictates that All Things Cute Must Die. Boy-and-his dog books are aimed at an older audience than Disney movies, but never mind that. The cuteness flux wanes as the target approaches puberty, but the end is the same.
Maybe the Disney Principle and the boy-and-his dog book dead puppy imperative are both special cases of something deeper and more sinister. Yeah, shmaltz, kitsch, whatever, but there is only one plot in a non-comedy story involving animals.
I mean hey, I don't much like dogs myself: They stink, they're dependent, they're born slaves, etc. Even so, they surely deserve to come into this world with some hope of living long enough to star in the sequel.
Hm, Lassie is an exception.