Instant Gratification vs. The Long and Winding Road
The thing about fanfiction is that it gives writers what the agonizingly long process of writing original-fiction-with-hope-to-publish doesn't: instant gratification. You whip up a story about ready-made characters fans of the canon (original source material) are already emotionally invested in, post it to a community like fanfiction.net, and BAM, an immediate audience ready to read and review. No need to wait on submission, publication, distribution, book reviews and advertising and publicity, all of which is a buttload of work and can take a minimum of a year or two to... infinitely longer.
Too Much of a Good Thing Can Lead to Doucheyness
But one of the greatest things about fandom is also the greatest cultivator of complacency--the fans. Don't get me wrong, I love a positive review. Who doesn't? Publishing is an extremely subjective business, but fandom and fanfiction is a million times more so because there are no gatekeepers and no quality control. I've seen countless fanfic writers post in their Author's Notes that they're open to concrit because they want to improve. But if a hundred people review a poorly written fanfic with glowing comments about how amazing it is, that writer who believed their writing needed improvement might begin to think otherwise.
The writer grows complacent, confident in their ability because, hey, all these reviewers said so and it's exactly what they wanted to hear. So when a reader comes along who takes that Author's Note to heart and leaves concrit, instead of reacting graciously, the writer responds with defensiveness and, in some cases, arrogance: "Well, all my FANS think my writing is awesome/didn't have a problem/haven't complained so who the hell are you to tell me differently? You obviously have no idea what you're talking about."
Concrit Is Not a Personal Attack (when it's actually concrit)
The thing about being an aspiring writer is that you must be able to take constructive criticism. It's a part of growing a skin thick enough to take all the rejection that will inevitably pave the way to success. You have to take criticism and rejection. You have to want to improve, to strive, to never give up.
Learn to be Objective
I'm not saying you can't love your writing. It's perfectly healthy to write something and be protective of it because of the hours spent laboring over it and the love and effort poured into it. But it's important to also acknowledge that it could be improved, greatly even. I look at my writing and I know, without a doubt, that there's a lot I have to work on. I rely on my crit partners to be honest with me, because I have trouble pinpointing my weaknesses myself. It takes quite a bit of effort, typically, to alter your style, to make changes that you maybe aren't comfortable with, in an effort to improve. So change can oftentimes be slow, painstakingly so. But is it worth it? Heck yes.
Being objective is hard, sometimes impossible, but try. If you become too enamored of your own writing, you risk becoming stagnant, which--by definition--is the greatest impediment to growth.
Fandom Fame Does Not Equal Published Writer
An amazing thing about fandom is that it opens you to this community of writers and readers, all of whom share a love of the canon. I've met some of my most enduring and best friends there. But I've also seen a lot of aspiring writers never break free of fandom. They've built a loyal readership, they interact and befriend their fans, they become "famous" in their own little corner of the internet. And for those in it just for fun, then hey, I've got no complaints. But for the ones claiming to be aspiring writers, it's easy to fall under the spell of perceived internet fame and forget what you began writing for.
I spent five years writing fanfiction, and I'm still divided about it. On the one hand, writing fanfiction--and, more importantly, reading it by much better writers--genuinely, truly, without a doubt improved my writing beyond anything I learned in college. I will always be grateful for that and to the writers in fandom who are some of the best I have ever read, published or otherwise. They inspired me to keep improving, because I wanted to write like them. On the other hand, I completely put off original fiction until the last two years when I looked back at my time spent in fandom and thought, "I've wanted to be a published writer my whole life. What the hell have I been doing?"
Fanfiction doesn't apply to everyone, of course, but the dangers of complacency is very real for all aspiring writers and the message is universal.
Never stop striving to improve. Never give up. Write, write, write ♥
My fanart of Naruto, the fandom I loved longest and hardest (possibly why he is in a state of undress *cough*):