I once spent a while staring at a rejection slip from the Michigan Quarterly Review, trying to work out whether the handwritten part said "sorry, we can't use this," or "sorry we can't use this." Was the pen mark after "sorry" a comma or just an accidental blot? This seemed significant because I thought "sorry we can't use this" implied genuine regret at having to pass on the story, while in "sorry, we can't use this," the "sorry" was clearly pro forma...
This level of attention to commas is sometimes appropriate, as when reading Samuel Beckett:
"I can't go on, I'll go on."
Before reading the delightful Beckett's Dying Words by Christopher Ricks, I wouldn't have noticed that the punctuation marks in that sentence each contradict the preceding phrase. The comma means "I'll go on." The period means, "I'll stop," and even presages death.
The punctuation marks in rejection letters, however, are quite devoid of import. So generally are the words. A rejection letter means "I have many stories to choose from, and my gut feeling is that I don't want this one." Since writers are so touchy, this is translated into clauses of disclaimerese: While we appreciate your sending us this, it does not meet our particular needs, but we wish you the best of luck placing it elsewhere. Please note that the magazines you submit to have no obligation to send you constructive feedback!
It helps to think of it as a numbers game. If you're expecting forty rejections, forty rejections won't bother you. Sometimes people read all my stories that are posted on online magazines, then let me know me which ones they liked most -- and my most popular stories with readers are seemingly not the same stories that received the fewest rejections.
Nowadays, when I receive a rejection letter on a short story, it impacts my mood for seconds rather than minutes. Sometimes the experience is even strangely affirmative -- I had the determination to send them this! And they were civilized enough to respond! Within two years! This is what being a writer means! So I'm doing it right!
Most writers have to learn to thrive on rejection. Beckett again:
"To be an artist is to fail, as no other dare fail."
I used to think this sentence was about some fancy metaphorical kind of failure. It is not.
Ursula LeGuin posted on her site this rejection letter she received on her great novel The Left Hand of Darkness. I suspect whoever wrote the letter just meant "we're not that into you," and would have done better not to try and rationalize things.
Don't misunderstand me: nothing I just said about embracing rejection means we don't also have the obligation to strive constantly to become better writers!