I once had a friend ask me why I stick with Writing. She said to me "there's a lot of rejection, and yet you pick up and keep going. Why do you torture yourself like that?" (She said this when I was coming off of a particularly crushing rejection from a major publishing house last year.)
My response at the time, was "You know what? I don't bloody know."
Rejection sucks, for want of a better word. Thing is, if you're a writer, and you don't keep your manuscripts to yourself, you will have experienced rejection at one point or another.
At one point, every time I got a rejection, I always questioned myself. "Why am I doing this? Why am I putting myself through this torture?"
The answer: because someday it will be worth it.
Writers are optimists. Sure, there are parts of the profession to be pessimistic about, not to mention we're human, for Cripe's sake, but whenever rejection comes, we have a day or two of self-pity wallowing, but then we're back on the wagon and going strong. You have to be in this business. Otherwise you have no business being IN this business!
Not that is doesn't hurt when rejection happens. It's a fact of life. And when it does, it tests you, it tries you, and you have to take the rejection and either A) learn from it or B) file it and forget about it and carry on. Throw yourself into other projects. Re-work your story and submit again. Even J.K. Rowling got rejected (and more than once) before she sold Harry Potter and the Philosopher's Stone. Rejection happens to everyone. Even the greatest writers through the ages have had to deal with it.
What makes us any different? Absolutely nothing. So keep plugging, and keep those chins up!