It can be depressing when no one takes interest, and a lack of response makes the writer question why they’re writing at all. To have one’s writing rejected is like you, yourself, are being rejected.
How do you end a story that’s not yours? Add another sentence where there is a pause? Infiltrate the story with a comma when really there should have been a period? Punctuate with an exclamation point where a period would have sufficed? What if you kill something breathing and breathe life into something the author wanted to eliminate? How do you get inside the mind of a person who isn’t there? Fill the shoes of someone who will never again fill his own?
Turning a manuscript into a book is easy; getting the manuscript ready to become a book is hard.