Not all novels are published as one book. Most famous example would be Tolkien's Lord of the Rings, which is usually called a trilogy despite the fact that it was written as one book. You could argue that is a forced trilogy, as it was published in three books because, as I understand it, it was too expensive to publish it in one volume. More recent we got George RR Martin's A Dance with Dragons, which was also published in two parts. In the end the decision to divide a book is usually based in economics, and occasionally, a publisher's wish to cash in. Which is where my current rewrite comes in.
For about two years, between 2007 and 2009, I managed to write a 350 000 word book despite the fact that many of my up-and-coming author acquaintances saw it as a bad move. Most of them advised me to either shorten it to 150 000 words (which is usually some publishers' limits) or divide it into two books. On the surface this may not seem like a problem. There is a relatively comfortable place within my book where the story can be divided in two but I need not consult Solomon to know that this division would be a bad (if reasonable) decision.
I believe my novel would not be broken if divided into two like some might, but on the other hand I feel the reasons not to are more compelling. One reason is that the book's emotional climax would fall in its second half, thus the second book. Even though there is an emotional climax within the first half as well (a rather devastating one for the main character) it would just make more sense as a whole and thus be more satisfying. Second, the climax proper is in the second half where many of the loose ends are tied up and the mysteries explained. I can easily manufacture a climax for the first half without it seeming tacked on (in fact, I was going to ad it in any case) but again it would not be 100% satisfying to most readers. Most would no doubt shake their fists at me for not resolving much. I do believe that they will care enough for the characters and their problems to read further, and the plots and mysteries should intrigue them too, but on the whole I'd rather give them more than less.
In the end (unforeseeable and undetectable) unless I go for self-publishing, then the division will have to take place. I am rewriting the novel in such a way where it can be done, but I will submit it first to publishers who would be more likely to not ask for it to be divided. It is important to me that my first novel should come out as best as possible and a division would undermine that.