I've read slush, sent form rejections, and the LAST thing I wanted was any reply from the writer.
What I did want was a publishable story. I promise, I won't remember your rejected work. When 50-75 submissions come in every day editors and agents do not want a time-wasting reply in our IN box that you understand our reaction. We assume you understand and will try-try again.
I was told by the senior editor to NOT include any comments to the writers, even if it might help them, because the writers always want to open a dialog and get more information about their baby. While some editors might have time for it, I did not. You take the piece to a workshop for feedback and make the next submission even better.
In one case I was on the receiving end of a rejection and it wasn't the standard form. The editor, who had bought other things from me, told me the premise was flat out stupid. I wasn't used to that kind of harsh, and it pissed me off. I'd have preferred a from rejection. Keep it impersonal!
I burned up my keyboard with several pages telling her how wrong she was, where to go and what to do when she got there, questioned her intelligence, taste, inability to see true genius, and concluded with the suggestion that she go suck a tailpipe on the nearest bus.
Then I printed it out and made sure I did NOT send it.
(Olden times, no email back then.)
I knew that sometime in the future I might be working with her again. No pooping in the pool!
Certainly you would never send a negative message to an editor/agent, but a positive message can have the same effect.
Write out your frustrations, never send them, then figure out why it got rejected, and try-try again.