Young people often arrive in fanon before they're very sure of themselves or their self-constructed identities. Fandom and fanfic allow them to work on self-identity by borrowing someone else's tools, tropes and symbols, rather than having to start from scratch. Prior to the internet, we did this by writing fanfiction in spiral bound notebooks under the covers by flashlight. Now, young writers come up and start sharing early. The fanon they build up is two parts canon and one part them. No wonder they feel entitled, defensive, possessive of a chunk of their own embryonic self-identity which is still in the process of being formed. They almost certainly don't realize that fanfiction/fandom is serving that function for them.

I definitely hadn't thought of this, and will be keeping it in mind for the future. It puts a different perspective and, as you said earlier, makes it easier to understand the 'why' behind the behaviour.

And I have absolutely no beef with "It bends canon, but I like this ship, so I'm doing it— read or not as you like!" Heck, that's what fanfiction's for in my opinion: exploring what we will not see in canon, in all shapes and sizes.

Thanks for giving me some more material to chew on!