I started a book and although I wanted it to be dark, will the readers find it too dark if the characters almost never win? Or are a lot of readers into that? In my book the two main characters' whole lives are terrible, but the point is they’ve been together through it all since childhood. It’s kind of like Forrest Gump (and Jenny) but without any comedy, is that too much?
Hmm. When I'm not sure about something, I like to look at some examples.
All Quiet on the Western Front has the characters never win. In fact, they all die, and their side loses the war (something we know from the outset, since that's Germany in WWI we're talking about). Nonetheless, All Quiet on the Western Front has its moments of warmth: there's camaraderie aplenty, there's gallows humour, there are the lighter moments with the French prostitutes and with the food. It's those warm moments that help the reader connect to the characters, and feel a loss when they die.
Dr Zhivago is not happy reading either. With Russian Revolution as the backdrop, and an intelligent, warm, honourable MC, it's set for tragedy. Every time it appears things are starting to go right, something happens that makes everything go terribly wrong. But there are those times when things start to go right. There's love, and there's the breathtaking beauty of rural Russia. Those draw you in, and keep you hoping, even as the MC hopes, for a better future, not just for him, but for Russia.
Finally, Hamlet, as an example of a tragedy. It starts bad, and it gets worse. At the same time, it offers plenty of wit. ("- What do you read, my lord? / - Words, words, words.") There's comedy (the gravedigger in the 5th act, for example), and there's Horatio's warm, unwavering friendship, offering some release from Hamlet's lonely lot.
So, it would appear that a novel can be quite dark, but light is still needed in it. It is the light that sets off the dark. It is the warm moments that make loss meaningful. There needs to be hope, for it to be dashed. And in real life too, there is light to be found even in deepest dark. In greatest hardships, people still find a way to love, to be friends, to give each other hope, to laugh at their executioners. I guess it's human nature to keep striving, and we expect stories to do the same - to keep finding a bit of light in unlikely places (until, if you so wish it, there is no more light to be found).
It depends on the book and the reader but broadly I believe it has to make sense either way. For example Nineteen Eighty-Four and Animal Farm by George Orwell wouldn't make the least bit of sense if the protagonists won, that's just not what they're about, they're both no win situations from the get go. Equally the protagonists of the Narnia books have God on their side, at least that was always my understanding of what Aslan is supposed to be, so they always win and it's only sensible that they do.
In most cases it's not so clear cut, you don't know until the bitter end who's won or lost, and in some cases, like the end of Prince of Chaos you don't even know for sure who won even after the story ends. This can be satisfying too though, because it makes sense within the context of the story that definite answers are beyond reach.
It's a matter of taste. The "A Song of Ice and Fire" series is quite dark and its main characters are usually having a bad time. Its books are quite popular, but I know people who stopped reading them because they found them kind of despairful.
To be frank, such a book would depress the heck out of me, but I'm just one person. If you're interested in taking a poll, then my vote is: give me some light! Life is already full of challenges and people (generally) use books to escape from their problems into a place where everything is sunshine and flowers (or at least could be if only the protagonist could get over his/her problems).
That said, there are 7,626,346,576 people (as of 15:38hr 2nd Jun 2018) in the world and so I always say that there's a market for pretty much any kind of writing. But I think that you might be heading for a niche with what you're proposing.
Tell you what would float my candle: Character A is having a really shoddy life (knock yourself out giving dark details) and just over the valley, Character B is having a different kind of horrible life (more detail to satisfy your urge for darkness). Then A meets B and somehow, by the magic of your pen, A's and B's misfortune comes together in such a way that they cancel each other out and (yep, you guessed it) we get a happily ever after to satisfy the reading masses.
As Galastel points out - let there be light (yeah, I'm paraphrasing). The above is just one way to provide that light whilst still giving yourself space to indulge your darkness-writing tendency. There are other ways.
Good luck going forward.
Check out The Chronicles of Thomas Covenant for a supreme example of this "genre". When I read the series, it was just the first two trilogies, and in writing this answer, I just discovered that Mr. Donaldson has since written a follow-up tetralogy. So the answer to your question is no, for gifted authors.
There are a lot of great answers here, and from my perspective they all have there own merit, but in my personal opinion, you shouldn't get hung up on such matters when writing a book. Most people have waaaaaayyy different tastes in reading, and there's no way to be sure if such a book would sink or swim. I'm an all genres type of person and enjoy both nuclear ends of the spectrum so I think long as the writer cares enough, the story will be good for what it's meant to be.
Realistically, if it's a passion project, I'd say you should just write your story the way you want and judge it for yourself, look at, and ask yourself "would I enjoy a story that is this dark?" and if you can honestly say yes to that, then you know someone else in the world is bound to enjoy reading it somewhere, whether they do so now, or 20 years from now doesn't really make a big difference.
However...if certified money and fame is all your looking from writing, then it'd be better if the book was out in a time when darkness and tragedy is in more in vogue. But trend surfing when writing can be really tiresome for some people, so it's all on you, and what it is you're trying to achieve with the book really.
Whether you decide to stick to your guns keeping it dark, or to bend your tale into something slightly more sunny, I do wish you and your book the best of luck.
The specific problem you're trying to avoid is called "Darkness-Induced Audience Apathy". If you feel like destroying your productivity for the next week, you can look it up on TV Tropes. People can (and do) get turned off a story if it's too dark for too much of the time. There are shows I've dropped in the past, and shows I plan never to watch, for that exact reason.
In your case, I think the issue is that your antagonist - fate - is too powerful. It's constantly beating your characters, time after time, for the entire length of your novel. As other answers have mentioned, that's just as boring and tedious as if the protagonist is all-powerful and wins all the time.
What you need to do is give the protagonists small victories that end up being entirely inconsequential. They succeed at something, or have something good happen to them, but it's quickly cancelled out by something else, or has no effect on how terrible their situation is, or in fact, they realise too late that it's actually made things worse.
Halo Reach is an excellent example of this. You know ahead of time that the protagonists will fail, and Reach will fall to the Covenant, but the game still gives you missions at which you succeed, because a game where you always lose is no fun to play. It's just that those victories don't matter. Most famously, one of your squadmates sacrifices himself to destroy a Covenant battleship... and then six more jump out of hyperspace. You win, but you still lose.
The key question is: Why should I care?
Part of the reason people like a book is because they get invested in the plot and the characters. They continue reading because they have a vested interest in seeing things play out. They care about what the characters do, what happens to them, how goals are achieved, how the antagonist is foiled, etc.
A potential issue with (bad) "dark" books is that there's never anything to get invested in. The characters are shit, their lives are shit, everything they try turns to shit in a rather predictable way. There's no reason to read on because there's nothing to get invested in.
Dark books aren't the only ones who suffer from this. "Happy" books can fall into a similar trap: the protagonist are perfect paragons, their lives are perfect, everything they do turns out perfect in a very predictable way. Readers don't get invested in those books either. The only difference is that writers are aware of that trap, and there's plenty of writing advice to make sure you have "conflict" in the story. Dark books, though, suffer from a misperception that "conflict" means "bad things happen to the protagonists", such that people mistakenly think a dark book has conflict by the very nature of being dark.
That isn't the case: "conflict" goes both ways. Yes, there isn't any conflict if the protagonist simply waltzes through the story, but there also isn't any conflict if the antagonists (be they animate or inanimate) just waltz through the story. If the protagonists never productively engage the antagonists (even in an inevitably futile sense), you don't have any conflict and you don't have a story.
Readers can get invested in a dark and tragic book, but they need some reason to care what happens. There needs to be productive conflict, even if the protagonists are fated to fail. There needs to be some reason to care about what happens to the main characters and their struggle - not like, necessarily, but care. There needs to be some mystery in how things turn out. Not necessarily if the protagonists will succeed or fail (there's a slew of books where the heros' eventual winning is never in doubt), but more of how they're going to succeed/fail. All of that is achievable with a "dark and tragic" book (as pointed out in examples in other answers).
A final note - you will inevitably get some subset of your readers who won't like your book because of the dark tone, or the fact the protagonists almost never win, or that it's lacking any "likable" characters, etc. That's okay. Not every book has to appeal to every reader.
I know that my answer might get lost in the hundreds of helpful answers above, but I'll just pitch in.
I find that if a story is too dark/sad, it gets a bit depressing. If the characters hardly ever win and the villain is always victorious, then it makes the reader want to throw the book on the floor with loath.
So here are some tips for your story...
- Don't make the story too dark that is, from a reader's perspective, miserable.
- At the same time, don't make your story so happy-go-lucky that the reader is bored. I've read some stories where the character is carefree, and it's boooring!
- Make your story exciting. If you're writing a mystery story, give a sense of suspense and adventure.
And most importantly...
- Keep the reader hooked!
I've recently finished reading Donna Tartt's 'The Goldfinch', which sold exceptionally well. It starts very depressingly, and things just get consistently worse. The moments of lightness are incredibly brief, there is no comedy, and everyone seemed to love it.
On the contrary, I think readers are more likely to lose interest if the work isn't dark enough. Barring young children, no one wants to read a book in which the heroes easily accomplish all their goals, the villains never really posed a threat, and nothing was ever really at stake. Obviously some readers will be turned off by constant, unyielding darkness, but constant unyielding success seems like it would be even worse. How do you have a meaningful central conflict if the protagonist never faces any difficulty?