Look, it's an interesting question...
I think, the more the character unfolds to us, the more we connect with them, the deeper we feel for whatever outcome they get through whatever ordeal they go through.
It also depends on the person...
I, personally, classify myself a dog person, my earliest memories have dogs in it. So, imagine, as I got my ticket to watch 'I am Legend' the other month, thinking "Haha, Will Smith in a stupid tryhard zombie movie, haha." Let's just say that I sure am glad mine wasnt the only cries I heard at the cinemas at what was in store for 'Sam'.
There I was, an adult man, bowling my eyes out at the movies. My friends laughing at me! Heheheh! Funny now that I think about. A Will Smith movie having that sort of impact on me!
And not to go back to the whole 'Animorphs-ending' argument but I think Daphnes summed it up pretty well. I too think the last few books lacked the 'magic' we all took the Animorphs for in the first half of the series. I am not sure if it was all just the Ghostwriters' fault. It was still KA's work. And she chose to end it the way she did.
If a book series can make a reader feel whatever it is that we each individually felt by a character like Rachel dying, it should be commended, for it's done what few series can do.
