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People buy books they way they buy vacations. The are looking for a specific kind of experience. There are certain elements that must be part of the experience. There are certain elements that must not be part of the experience. A certain amount of surprises and novelty are welcome as long as they are the right kind of experience. 

The precise kinds of experience that fall on the must and must not list vary from person to person, so genre specifications are not uniform. Some must have a particular setting; others don't care where it is set. Some must have a particular kind of hero or heroine; some welcome variety. Some must have a particular political bent; some don't want politics at all. Some must have horses, some must have dogs, some must have aliens, some must have sex, some must have werewolves, some must not have spider or snakes or violence or religion. 

A genre is, in other words, an arbitrary bundle of desires and aversions shared by a sufficiently large number of people that it is worth writing books just for them. 

Either your book checks all must have boxes and must not have boxes for a particular genre, or it does not. Conceivably it could check all the must and must not have boxes for more than one genre. Vampire romance, anyone?

Deciding which genre to write to starts with the author. Either you decide to write your book to the specifications of a particular genre, or you decide to write it first and figure out the genre later, or you decide to ignore genre and hope to be the breakthrough genre-busting novel that defines a new genre around itself, -- Vampire romance, anyone? -- or you decide to go mainstream/literary and hope to appeal to readers that like more than one kind of book and are a little more catholic in the experiences they seek. All of that is on you.

Don't fall into the trap of assuming that any book you write must inevitably have a genre. The genres are particular and there are lots of book, and potential books, that simple don't meet the requirement of any genre. You might as well try to sell vacations to Gary, Indiana. 

A publisher or agent will want to know what genre a book is. If it fits more than one, they will decide which of the available genres to sell it in. But genre is an expression of reader taste, and publishers and agents follow genre, they don't lead it or invent it, though some may discover it. A genre is just a sufficiently lucrative grouping of taste, and and agent or editor who discovers such a grouping of taste (usually by taking a flyer on a book they really like that does not fit a currently defined genre) can build a career on the discovery. But that is a risky move, much more likely to produce a flop than to found a new genre. Don't count on finding too many agents or editors ready to do that for your book.