creative writing - Do readers not like a book if it's too dark and the characters almost never win?

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?