Hmm, now there's an interesting question. I was having a nice long conversation with someone today, all about why the ending of Mass Effect 3 was such a colossal bag of donkey doings, and said person asked a question about whether a bad ending can really ruin all the good stuff that came before, when the entire experience up to that point had been good. My immediate was a big fat yes, but it raised a rather intriguing question: why? How come the ending has such enormous power over a story, and thus, how is it possible that a bad ending can ruin everything that's gone before?
Might it be something about how we, as humans, are hardwired that makes the ending of the story the single most important bit of the whole enterprise?