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Because you can't see your own mistakes. You know, in your head, what you want your story to accomplish. You know who you want to end up with whom. You know who you want to punish, and who you wan...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/1717 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/1717 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Because you can't see your own mistakes. You know, in your head, what you want your story to accomplish. You know who you want to end up with whom. You know who you want to punish, and who you want to see succeed. You know which characters you like and which are your villains. But the challenge is writing your story so that anyone else who reads it sees the same things you do, without having you there to explain it. You need to get everything out of your head onto the page, and convincingly. A good editor will help you to shape what's on the page so that it guides the reader to see what you want the reader to see. A good editor will also point out where you made the heroine a wimp, the love-interest so perfect that he clearly leapt fully-formed from the brow of Tiger Beat, the villain into Snidely Whiplash, and how the final twist can be seen a mile off. Without an editor, you won't necessarily see those things are there, or you won't think they need fixing. The editor is there to bring out the best in your work.