I love stories ending happily, but sometimes I can just as equally love a story that has an unhappy ending. My biggest thing is that the ending has to fit the plot, and most of all, be satisfactory.
By that, I mean that you have a reader who is following along with this plot throughout your story, and they may/or may not know that it will end unhappily, but they continue to read to see how it plays out. You HAVE TO give them an ending that feels like an ending. One that doesn’t leave them frustrated for wasting their time, or upset that this tragedy came out of absolutely nowhere and makes no sense. It needs to be well executed and tick the boxes.
I read one story on the app shortly after joining, and it was a long story, but it was good. However, we were, as an audience, lead to believe it would end happily, then right at the very end, it just had the two characters going their separate ways, both broken hearted.
That was hugely irritating to me, and several friends I know who also read the story as it just came out of nowhere and left us questioning what the motives were, why it happened and what the point was.
The whole thing had been dampened by this ending that felt like it didn’t fit, and was hastily executed for impact. So I think if you’re writing an unhappy ending, it should be well thought out, and checked to make sure it is in line with the entire story, not plucked from mid air, and feel complete when it’s over.