At times it was only the fact that I was 400 pages into it that made me keep reading. Not because it wasn't beautifully written and a page turner in the character-reveal rather than plot-reveal sense of the word — but because the characters must endure the endurable. (And somehow, most of the time, they do.)
The book grapples with big questions, some of the biggest. Why are we here? How do we find meaning? What are the limits of love?
It doesn't answer any of them, of course, but it makes us ponder them, and it makes us care about the characters who are pondering them. And that, as good fiction always reminds me, is what it's all about.