I’ve read some fan fiction. I mean, you have to; in the literary world, you have to actually read something to know about it. You can’t say, “I know about Dostoevsky’s Crime and Punishment” without having read it, in the way that you can say you know about cars without actually having built one. Although, I’m sure some people will quibble at throwing the umbrella of literature over fan fiction, but let me be pedantic and point out that I never said good literature.
Don’t get me wrong; I don’t think that fan fiction is, per sé, bad. Fifty Shades of Grey could be considered good if you only use sales as a metric (though, admittedly, I haven’t actually read it). The problem is that when I read/hear/watch/otherwise absorb into my consciousness a story, it speaks to me in a certain way. Fan fiction takes whatever someone understood of a story, a character, and runs with it.
Because an author has to cull such parts of his idea that don’t fit into the story, we as readers fill in the gaps with our own ideas and creations. That’s the beauty of reading, especially; we are free to visualize the scenes in our favourite books how we like (and why there is so much discord when a book is rendered in film because the characters don’t “look right” or “act like in the book”). This is why some people can love a character while others despise it; each reader fills the voids in the character’s make-up to flesh out what they think works best.
Of course then when someone writes a work of fan fiction, they layer these interpretations and constructs over the character, idea, scene or story. Because I, personally, visualized the scene differently, or understood a character in a different way, these “changes” appear jarring, inconsistent. I’ll accuse the writer of not capturing the “essence of the character”; that part of the character that’s not explicitly written in the story but that I, personally, understand. That every reader understands…differently.
So it’s not that fan fiction is necessarily bad (though some if it is honestly atrocious), and I don’t criticize it for lack of originality. Every author since the second story was told has used elements from other artists to weave their own tale. I’ve seen plenty of stories that I felt were great ideas, that characters showed enormous potential -and then the author didn’t fulfill my expectations. I’d like to see more depth to secondary characters in my favourite books. Those are motivations for writing fan fiction (and I might have entertained the idea myself).
But ultimately, you’re just writing your opinion. It’s a valid opinion, but I and everyone else disagree with it. If you don’t like fan fiction, consider it beneath you; try reading some about characters in a book or show you haven’t seen yet. Then you’ll see whether you dislike it because it’s truly bad or because it’s disagreeing with how you imagined something would be.