I am not saying that everyone in the world needs to write, read, or adore fanfic. For one thing, I don't think there's any truly universal "everybody has to do this" experience, except for maybe peeing. Everybody pees, right? For the purposes of this essay, everybody pees. (If you do not pee, please don't tell me.) So no, if you're not into writing fanfic, I am not judging you. I am not saying you're not awesome. If you're not into reading fanfic...I think there's a very good chance your definition is too narrow. Shakespeare, as I have said, wrote fanfic. Virgil. Bridget Jones's Diary and Wicked are both alternate universe, or "AU," fanfic of other people's properties. 10 Things I Hate About You and Easy A, also basically fanfic. Any company or studio-owned property where the original author is no longer in charge? Kinda fanfic, on a lot of levels.
You do not have to read Harry/Draco romantic coffee shop Little Mermaid AUs to be reading fanfic. Fanfic is everywhere. The fact that some of it is based on things that have slipped into the public domain does not make it any less fanfic: it just makes it fanfic that can be sold for a profit.
In recent discussions of fanfic, I've seen several people say things along the lines of "but all fanfic is about sex" and "it's not okay when people write relationships that don't exist in the canon." (Neither of these is an exact quote: I am paraphrasing from multiple sources, in part because I'm not trying to call anyone out in specific, and in part because these are sentiments that I've seen repeated on almost every discussion of fanfic. I think that it's something we need to talk about.
First up, "all fanfic is about sex."
This is patently untrue. A huge amount of fanfic is about things other than sex. I read a novel's-worth of fanfic every week or so, and I very rarely read explicit sex unless it happens in the context of a long, long story about other things. Most of my favorite fanfic would not be rated above an "R." Much of it wouldn't get above "PG-13." But let's assume, for the sake of argument, that 50% of fanfic is purely about the sex. 50% of fanfic, however well or poorly written, is only there for bangin', clangin', and bringing down the house. Okay.
Erotica is a big deal. Porn is a big deal. There's a huge amount of it out there, and there's a whole industry dedicated to making porn parodies of popular properties. XXX-Men and the Anal Avengers and Wet Dream on Elm Street, these are all things that exist. They're basically thinly veiled visual fanfic, much of which is incredibly male-gazey and heteronormative and gives us the superficials ("A dude dressed like Giles is going down on a chick dressed like Willow in the latest installment of Muffy the Trouser Slayer!") without giving us the things that draw us to the property in the first place. "Porn with plot" is a huge fanfic category because people want to write some fun erotica, but want it to actually be about characters, not cheap cosplay with enormous primary and secondary sexual organs.
Some of the pairings fanfic authors pursue will make some people uncomfortable. I used to be heavily involved with Supernatural fandom, and I hated the most popular pairing, which put two brothers together in sexual situations. But the nice thing about fanfic is that responsible fanfic authors will tag their smut. So if I clicked a link for a piece of fic and saw "M/M, Sam/Dean, Incest, NC-17" as the tags, I knew that I needed to nope on out of there. The authors, who hurt no one, got to have their fun; their readers, who hurt no one, got to read some awesome smut; I went and found stories about Dean and Jo being awesome, often in a fully platonic way. Not everything is for everyone.
But okay: some people don't want to read about non-canonical sex. That's cool. Just don't click that link, and you'll be safe from all the porn. Porn cannot in fact follow you home.
Note: I am not saying that you need to start reading fanfic porn. No one is required to read anything, unless they're taking a class. (If you do not want to read fanfic porn, avoid courses titled "Tranformative Erotica in the Copyright Age.") But using "there is porn" as a way to shut down all fanfic is reductive and unnecessary, and what it shows is not a clean-minded commitment to the canon; it's a refusal to consider that there could be depth and meaning to something that is hugely important in a whole lot of lives.
Secondly, we have "it's not okay when people write relationships that don't exist in the canon."
I want to tell you two very important things that it took me a long time to realize. The first is that if you're talking about something like a TV show or a comic book or a movie, the odds are very good that those characters, and that canon, were created by committee. You will have multiple people putting words into the same mouths, and while there's generally an effort made to keep characterization relatively straightforward, at the end of the day, each of the real people will have a different version of that fictional character in their heads. Take, for example, Usagi Tsukino, better known by most people as "Sailor Moon."
Usagi's role on the show is to love. It is to be a soldier of love, a warrior of love, someone who will die for love. She is love all-encompassing, love without limits. Yeah, she's a little immature sometimes, but that's okay, because she loves you.
My friend Nikki is doing an end-to-end rewatch of Sailor Moon. In a recent episode, there was a scene where Usagi, upon being presented a love letter from a girl, said "I'm taken," which is totally great and good, and then "But you should fall in love with a boy, it's way better" (paraphrase). Um, what? This is a girl who has died and been reborn for love, who is in awe of the relationship between two female friends, and who has never said anything like this before. It's just a little dagger of heteronormativity coming unexpectedly from the mouth of the living avatar of love. (Nikki points out that Usagi did say something similar once, about a hundred episodes earlier, before she really got to know Haruka and Michiru, and that both times, it was done purely for comedic effect. So take that as you will.)
It's canon. She said it on the show. But it's not a thing my Usagi would ever say, and it may not be a thing that Naoko Takeuchi's Usagi would ever say. Naoko Takeuchi created the franchise. She did not write that episode. Do you see where I'm going with this? It is entirely possible to have things in the canon that contradict the character as it was intended.
The second is that creators can be wrong. I have written scenes in various books and stories that were pointed to by my beta readers (thankfully) as being incorrect. Sylvester wouldn't say that; the Luidaeg wouldn't do that. Because I have good betas, I can fix those things, but there's always the chance that someday I will go entirely off the rails. I will kill a character because I am mad at a real person who loves the fictional person I have created. I will write a final chapter before I actually begin the series, and refuse to revise it. I will do something that makes my most loyal readers go "whoa, fuck, back it up babe."
Some of them will back it up with fanfic.
Note that this only addresses creator failure. With shows and movies, there's real life to be considered as well. I know a lot of Glee fanfic authors who have gone fully AU following the deal of Cory Monteith, who played the romantic lead. They don't want to continue the series without him, and I can't blame them at all.
Now that I have told you these two very important things, and their sub-things, here is another thing: who cares?
When I watched Kim Possible, I always expected Kim to wind up with Shego, not Ron. I believed that Myka and H.G. were meant to be (I still do). I looked at Parker and Hardison and Elliot and saw a family unit that was never going to give a damn about who was sleeping with who (although Elliot would probably tell you that Hardison had a very distinctive snore, while sleeping with Parker was like sleeping with an abnormally large housecat that hogged the pillows). In the first, it was a cartoon; never going to happen. In the second, I'm not sure why the writers made the choice they did. In the third, I got my wish. But if I hadn't, the way I viewed the show would have remained entirely valid.
I get checking out when someone is writing a pairing that doesn't work for you. My dislike of Sam/Dean in Supernatural has less to do with them being brothers than it does to do with the fact that I just can't buy them as a couple. But sometimes couples have surprised me. I just finished reading an epic-length Harry Potter fic where Neville/Padma was a primary pairing, and I would never have expected it to be awesome, and it was awesome. There is room for beauty outside the canon, especially when the canon is the work of many hands.
Especially with older fandoms, or with pairings that involve a lead character, straight is an assumed default. Well, statistically, that's not right. One in ten. So that's one Ensign, one Companion, one whatever in ten, minimum, who shouldn't be straight. When I'm writing fic about a show that didn't give me the representation I needed, what's wrong with wanting to see it? (Before the inevitable question: the reason writing Buffy/Faith is applauded, and Tara/Xander is likely to get you a frowny face, is that same issue of representation. One in ten. Currently, we have less than one in fifty on a canonical level. Taking it away because you want to show that it's an equal playing field is not creating an equal playing field. It's being a jerk. Unless that is the only story you want to write, please, consider the numbers, and don't.)
I'm not expecting to convince anyone who's already decided fanfic is awful that they should give it another try; some ships have sailed. But I am asking that you consider the arguments you hear about it, and consider why they might be flawed. There's nothing wrong with writing a little smut. There's nothing wrong with having your own view of the canon, especially when that canon goes on for more than a single book. And there's nothing wrong with seeking representation where there isn't enough.
These are all reasons that fanfic is glorious, and that I am so, so glad to be a part of the fic community. I always will be.