Editors are often sympathetic, patient people at heart (though overworked), but I know they all wish that more people understood that when one asks one's author-friend to do a favor and send a manuscript to their editor, one is not asking the author to do you a favor... What one is really doing is asking the editor to do you a favor, and you don't get to ask strangers for favors.(Emphasis mine.)
Publishing a manuscript is about appealing to thousands of people. There are lots of wonderful manuscripts that would make dozens, or scores, or even hundreds of people happy. They can't be published. They would be cherished, and they would sell, but they would not sell enough.
Publishing is about appealing to strangers.
That last line has a corollary I think helpful to anyone whose heart is breaking because they can't get the book they wrote--and love like it's their child--published:
Yes, your book is valid and worthy and precious, but those things don't guarantee it's saleable. To be saleable, your book must appeal to strangers--lots of them.