C. S. Lewis addresses why he gave The Great Divorce its title in the preface of his book. He states that it is in answer to William Blake's book The Marriage of Heaven and Hell, but whereas Blake argues that heaven and hell are the same place and that there is no such thing as good and evil, Lewis argues that there is an "unavoidable either-or." He agrees that people will almost certainly experience both states during their lifetime but says that at some point they will have to make a choice.

We are not living in a world where all roads are radii of a circle and where all, if followed long enough, will therefore draw gradually nearer and finally meet at the centre: rather in a world where every road, after a few miles forks into two, and each of those into two again, and at each fork you must make a decision.

C. S. Lewis calls the choice we have to make between good and evil the Great Divorce, meaning that Heaven and Hell are separate places. However, he does call it a choice and states that

I do not think that that all who choose wrong roads perish; but their rescue consists in being put back on the right road.

In the book, the dead are given the choice between staying in Hell, which closely resembles what they knew on Earth, or going up to Heaven. The majority choose Hell because it's something they already know and understand. For C. S. Lewis, this is a great shame. He states,

I believe, to be sure, that any man who reaches Heaven will find that what he abandoned has not been lost: that the kernel of what he was really seeking even in his most depraved wishes will be there, beyond expectation, waiting for him in "the High Countries".

I agree with the comment below about divorce. If you want to explore this topic further as it relates to the Bible, remember that Malachi 2:16 says that “God hates divorce" because of all the pain it causes and, in Biblical times, how destructive it was to women. God allows for divorce. Even in Biblical times, he allowed it to protect women from the whims of men who could just set them aside when they got bored. Since divorce means "separation", the GREAT divorce is man choosing to separate himself from God, which results in loss of eternal life, in the Christian view.

 “The Great Divorce,” by C.S. Lewis was published in 1945.  Mr. Lewis informs the reader, in the preface of the novel, that he is intending to “combat the universalist idea that everyone will be saved, in the end.”  The novel is written as an allegory and has been compared to other novels such as Dante’s “Divine Comedy.” 

The premise of the novel is written about characters, who are damned, being transported, on a bus, from Hell to Heaven.  The Narrator tells us that they are given an opportunity to stay in Heaven with God.  However they reject the offer, because they decide they would rather stay in Hell.  As the reader continues on the ride with the travelers, on the bus, we come to see the divorce, in the title, as man’s choice to separate himself from God; to consciously choose evil over good.