In fact, I've loved it for decades -- long before it was even called 'fanfic.' (Back when it was shared via Zines and so on.)
And I've been involved in tons of fandoms (and have written tons of fic). And I've READ a ton of it, too!
There's BAD fanfiction out there -- but there's tons of BAD fiction, period, in the world. And I find there's fanfic I love as much (or more) than fiction.
Some of the BEST writing I've ever read in my life has been fanfic. And folks, I read CONSTANTLY. I'm someone who is almost always in the middle of 5-6+ books (not counting manuals and non-fiction) at a time.
I'm surprised by people who hate fanfic. But I figure they haven't actually read the good stuff that's out there. It would be easy to miss, considering how much fanfic exists in the world!
Writing fanfic is how I started to really improve as a writer, by the way. If I ever get a book published, I'll owe it in part to my fanfic writing.