Writers -- like all other human beings with feelings -- sometimes act as though their words are inviolable. As a result, the editor (who has tentatively won the trust of the author) must tread lightly but with determined purpose to get the text "right."
If it's only punctuation and spelling, the task of proofreading is relatively simple; however, general rewrites that involve restructuring are more complex in that the editor must retain the author's voice but render the work into standard English. Of course, the writing of narrative differs from that of dialogue. I'm editing an entire work now that must be proofread before the second printing, as much was missed in the initial reading (yet, it went to print).
The point being, the writer may have wonderful thoughts to share, but definitely needs outside, professional assistance. If he knows this and acquiesces, it's so much easier to put his promising work into language both understandable and enjoyable to read . . .
CC