This is where the "thick skin" part of your must-have list comes in handy. Every writer who braves the literary world and submits her work will, at some point, get a rejection on her manuscript. Sometimes it's a pre-printed form letter explaining that the agent/editor appreciates the opportunity to review your work, but your manuscript wasn't right for him, and he wishes you luck submitting elsewhere. Sometimes the agent/editor will take the time to add a handwritten note to the bottom of the letter and go into more detail on what they liked or disliked about your work. That's a big pat on the back! Be sure you send him a thank you note for his trouble, even if you get only a form rejection.
If your work is rejected, don't despair. Every writer gets rejected sometime - even the bestselling ones. And remember that they had to start somewhere, too. Allow yourself a few minutes to wallow in disappointment, and then get back to your work. You'll never sell if you get discouraged and quit submitting. Victory will be that much sweeter if you don't give up. You may not sell your first manuscript, nor your second or third, but you can only hope to publish if you keep working on your craft and improving it with each attempt.
Rejections can result from a number of things:
* The agent/editor's personal preference (they just don't like the kind of story you offered);
* Poor plotting/characterization/story mechanics;
* Poor punctuation/grammar;
* Wrong genre for that agent/house (you'd be surprised how often this happens - research before you submit!);
* They already accepted/published a book similar to yours.
There are many more reasons a story can be rejected. Don't give yourself a headache trying to decipher a form rejection. Throw a party if the agent/editor gives you reasons for his choice in not accepting your work. You may find a snag in your manuscript you hadn't seen before, and a chance to fix it. Always continue to hone your craft, and someday you may get that long-awaited phone call with a yes.