As the professor repeated during the movie's course, the ending was perfect with Harold Crick's death as it was. However, Crick learned the truth about his life and how it was going to end and tried to talk Eiffel down from it, so Eiffel gave him the complete work to read. Upon finishing it, Crick saw her perspective (as well as the Professor's) and decided that he really should die to save the child. His approach of Eiffel, agreeing that he should indeed die, was that which changed her mind. As she said, a man that knows his death and is willing to face it for the reasons he did is a man that she would want to live. So before finishing the final draft of the story, she changed it. The professor held his original position, pointing out that it was now only "OK", and not quite the masterpiece of the original. Her argument was that now she would go back and change the story to match the ending accordingly, as one could assume, to properly capture what happened from Crick's perspective, and in essence, justify his continued existence and her decision.
Harold loved Ana and did not want to burden her with the knowledge that he was going to die. He had come to fully appreciate life and wanted to enjoy his last moments. If he had told Ana, she undoubtedly would have tried to talk him out of it, which would have made their last moments together unpleasant. Additionally, although he had come to accept his fate, Ana would not have understood his decision.
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