Searching for well written stories on fanfiction.net is usually like looking for a penny inside a planet-sized pile of rancid maggots. And even the minority that aren't obviously written by illiterate thirteen-year-olds tend to be pretty 'meh' for whatever reason, be they insipid 'romances' completely inappropriate to the universe's story premise; unimaginative plots; full of awkward phrasings; blatantly out-of-character behavior among the leads; or just plain boring rot in general.
And yet it's really the only place to find anything at all for a lot of fandoms, if you're not involved in the online activities of others and know where else to look.
I occasionally write myself, though I'm certainly not what you'd call prolific. I don't claim to be the best writer ever, not by a long shot, but I at least put a lot of effort into characterisation and making sure everything feels 'authentic' to the premise of the world. But sometimes I really think I must be one of only 5% of people who actually bother with any of that, or even has an understanding of basic grammatical principles.
Fan fiction tends to have a pretty bad name among 'normal' folk because of these things, and quite understandably so. But that's a blasted shame, because some of the good ones you can find by looking hard enough are actually at least as well written as professional published works, and sometimes even better. Talk about having a frustrating hobby.