Okay, first of all, saving trees is a good thing. :) But that's not what I mean. All rejections, paper or otherwise, have a positive side.
1) A rejection means you're in the game.
Lots of people talk about getting a novel published...someday. They cite the dream of holding a book in their hands (or on kindle) that will touch other people's lives. Well, talk is cheap. You, my friend, are not content with talking--you are a DOER. Getting an R means you worked your ass off to learn to write well and then took the leap to submit...something that should be celebrated!
2) A rejection means you believe in yourself and your work.
Unless you're a masochist, a rejection's cheese-grater-on-brain feeling isn't something one braves just for giggles. Everyone in this business knows publication is not for the faint of heart. When on submission, you lay yourself bare and say, "This is my work. I believe in it, and so should you." The fact that you are willing to take the rejection hit SHOWS how much passion you have. How many other people can say they feel passion for their jobs?
3) A rejection is an opportunity to learn.
Each rejection, even a form, is a chance to re-evaluate what you're presenting. Look at the materials sent to this agent or editor and put yourself in their shoes. Why did they pass on this? Is the query streamlined and voice-y? Does it contain a compelling hook? Is the writing solid in the sample you sent along--strong characterization, interesting premise, hooks to inspire the reader to keep reading through those first pages? Or are you relying on description or nice writing to pull them in and keep them going until you get to the good part on page 10?
Let's say you decide the query is solid and the writing sample's a shining monument to awesome. Ask yourself then if you targeted this person effectively. How much research did you do before hitting send? How well did you know their interests, their recent projects and authors' work? Did you see they take YA in a forum somewhere and so fired off a query, without checking current websites/blogs/interviews to really understand what projects they want most?
4) A rejection is a challenge to do better.
Writers are fighters. If we weren't, we'd choose some other profession with friendlier odds. Take the frustration over a rejection and challenge yourself to prove the Rejecter wrong. Turn a critical, honest eye to the material, evaluate, and PAY ATTENTION. Is there an above question that makes you feel a momentary flutter of doubt? If there is, chances are this is an area to focus on before sending your work out again. Let the fire of wanting to prove yourself be the motivation to strive for your very best. Don't settle for 'feeling pretty good about' any aspect of your work or query. Be satisfied only when you feel you have done everything in your power to ensure success.
Rejections come with the territory, so try not to take them personally. Quality work and careful targeting are key, but it still comes down to a personal preference, something out of the writer's control. Submission is like marriage, and it takes time to find the right person who will fall in love with your work.
Make it your goal to feel confident about your work and the rest will take care of itself!