If there's one thing would-be writers are never prepared for, it's chronic rejection.
The key word in that previous sentence, of course, is 'chronic'. All but the softest petals in the big garden of life can take the odd "this doesn't suit us" email. Anyone who goes into this writing caper with their eyes half open will expect it. But it's the endless grind of rejection after rejection that separates the wanna-be writers from the will-be authors.
As of this writing I have published about 20 pieces of fiction in eight years. Not particularly prolific compared to many short fiction authors, but not terrible, either, considering I'm not the obsessive type. (I write, and write hard, when the inspiration strikes me, and I put plenty of work into editing once I'm done, but I'll sometimes go two or three months without striking a key.) My point is this: while I've had 20 stories published, I've probably had 20 multiplied by 20 rejections.
Some people - mainly those who think writing looks like a nice cushy way to make a living - can't take this sort of endless negation. To get through it, a person needs to a) love writing for the sake of writing and b) have enough innate writing talent to create something an editor will want to publish.
While I don't know this for a fact, I suspect each writer has his or her own method of coping with yet another soul-crushing "this was well written but didn't quite fit our needs" rejection. A habit I've developed over the years is to scan the first paragraph of the email and if I see any of those familiar yet ominous phrases, I stop reading and go do something else. This prevents me from dwelling on my disappointment when it's at its rawest.
Within an hour or two, I've usually become more philosophical about the rejection (unless it's a shortlist rejection - those take longer to get over) and can bring myself read precisely why the editor or slush reader nixed it. More often than not, these days, it's an unenlightening form response. Occasionally the remark will expose the reader/editor as a nitwit who missed the point of the story entirely. But now and then, you'll get some advice as to what was wrong with your story. If you're a new writer, especially, this advice is golden. I can think of at least one story of mine, 'Trouble With the Locals', that became publishable when I accepted and applied a slush reader's critique that it was way too long.
Like an injection, rejection never stops hurting. But like an injection, sometimes it can be good for you.