While I was writing my novel I had problems deciding the fate of on of my main characters. I had one of those holy crap moments where everything fell into place (in my head lol) and I loved it. I told someone and they said "I dunno...maybe write it since you were inspired but maybe from this point do the other thing too and see which you like better." Well I didn't so both, I went with the one I thought was best and loved the whole thing. Now that I am done and gave it to be proof read, the girl who read it said, "I love the story, accept what happens to Dave, it should have been this way..." and gave an example which was the other idea I had but didn't like as much. So now I am doubting my choice and confused. I read in a post that someone said- have someone read it and listen to what they say and do it... But I don't know now. I like the second outcome in a fairy tale sort of way but the story is dark and basically shows how someones life can fall apart by one wrong choice. Do I keep it how it is, hard and effed up but real. Or make it so people will go- yay! and be unrealistic (and not as long because 1/3 of plot will go poof) ?? This may not be the best example I guess but the movie The Mist, I was the only one who loved the ending. I loved that it was so dark and I was left saying "OMG THAT WAS F*CKED!" because even though it was a supernatural situation, the feelings and the actions were completely honest and real. But I can also see how people want the fairytale endings. I don't know what to do now.
When I read a novel, watch a movie, or follow some other narrative I usually identify with the main character. Some part of me becomes that character and I see the artist's world through its eyes. Naturally, I feel that character's suffering and rejoice at the character's triumphs. When the character fails, I feel disappointed. However, while I might curse the author for a bad ending, it might still benefit the story. After getting over my initial melancholy, I might see the tragic beauty in it. I might praise the author, seeing how a positive ending could've damaged the entire piece. I might even thank her for sparing me a sickeningly sweet cliche. Personally, I dislike utter tragedies and said sweet, cliche endings often annoy me. I like bittersweet endings the most and enjoy focusing on the good, however small, that has come out of the whole ordeal. Others might react differently. Some might hate tragedies, never able to get over the initial blow. Others might not bat an eyelash at despair or simply revel in it. From questioning friends about the subject, I've noticed that this is something many feel differently about. I've also noticed that infrequent readers prefer more 'feel-good' stories, while avid readers might enjoy the occasional tragedy. What I'm trying to say, in a very roundabout way, is that different people have different tastes, but tragedy is definitely one. If you want additional opinions about the ending, consider posting it here for review. If you can't, just ask someone else you know.
I lean toward Stephen King's advice on this: if one person says it, take it into consideration, but you're not obligated to take their word for it that this how the story should go. However, if you have several readers commenting on the same issue and they're all in agreement, you'll want to seriously consider revising that aspect of the story. However! Just because an issue needs to be revised does not mean you need to take a reader's suggestion on how it ought to be fixed. It sounds like in your case, the reader doesn't like the unhappy ending. Rather than completely changing it, you may leave some slight ambiguity, or a glimmer of hope buried somewhere. Or not. Not everything has to have a happy ending.
You are the writer. Things should happen the way you want them to. Frankly, if something happens to one of your characters that really irks a reader I say, well done. I like it when something I write inspires any kind of emotion.