I could play cool, and pretend it doesn't hurt to receive a rejection slip from an editor.
But it does. It always hurts every time... like having your heart carved up with a rusty nail... like hearing your parents say "We wish you'd never been born!"... like having your legs run over by a truck while you hear Britney Spears singing "My Prerogative."
It softens the pain, though, to have an agent who receives the rejections and forwards them to me. (Thanks, Faye.)
When an editor rejects a manuscript, the worst part is knowing how arbitrary the publisher's review process really is. There is no objectivity.
You can polish a manuscript all you want... in the end, you may still be treated like J.K. Rowling, who was rejected by nine editors who thought Harry Potter And The Philosopher's Stone wouldn't sell. (Listed by The Sun as one of the 50 Worst Decisions Ever.)
If you bear in mind what happened to Rowling, your rule of thumb should be: Don't give up your manuscript until it has been rejected at least 10 times. Then you shelve it and start writing something else.
If you want to read other writers' acounts of rejections-before-success, this article has several.
If you just want to wallow in the misery of other writers being rejected, there is the Rejection Collection.
But why do that? You should rejoice in the regret, self-loathing and bitterness of the editors who rejected J.K.Rowling. Every time you meet a publishing professional, ask which person rejected Rowling. And if you ever come face-to-face with one of the editors who decided the rejection, ask him or her:
"How does it feel to be such a f***ing jerk?"