Happy Endings in Romance Genre - Must it be ....?

I am well into writing my book in the Romance Genre (I also have another in the pipeline).  As I am also an avid reader I have to admit the desire, no matter what happens throughout of "that Happy Ending".  The question is - is it essential to success?  Is it actually what the reader wants, who has to face reality every day where it is most certainly not always the case?

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In my writing, however, I have a choice.  Whereas a writer throughout the writing process feels so much for her characters the question is : should one succumb to that happy ending?  So many great stories in real life have ended so differently.

My conclusion on the debate is : it depends on which story one is writing and the reader should always be kept guessing!