I wouldn't want to know the answer re human characters, although I have a general preference for happy endings. However, with cats (and other animals) I get very upset if they die at the end of the book. I also feel cheated: I've been lead on a story journey with this cat, invested a lot of time being with him, only to have the author end the book on a sour note. If it was a paperback, I'd throw it at the wall.
I'm very fond of cats in particular.
In the past, I've read books based on real animals which have this fault. Okay, it happened, but I want to put the book down with a smile on my face.