This has come up a few times in the Sonic discord I moderate for, and has already caused incidents with others because nobody understands a person who does not like getting free art, so here's my honest reasons.
I am extremely picky about how my characters look. I have a very particular aesthetic in my head, and only I am allowed to fuck that up when my incompetent ass puts it to paper.
At least then, I have no one to blame but myself when everyone has the wrong impression of how they're supposed to look. I am too inadequate an artist to make accurate reference material, and the pressure to draw more often for the sake of creating some gets to me. (The pressure to draw in general in a community of artists is already eating at me enough.)
It's nothing personal, and this isn't meant to be a damnation of the work others do. These are my own personal expectations with solely my own characters.
And every time I need to ask someone to make a change to a character in gift art, my anxiety shoots through the roof even if I know they will oblige.
In the past, other people have been so shitty to me about me asking for changes that it stuck. I've even had people retaliate by drawing my stuff wrong on purpose just to spite me.
I have been called entitled, a choosy beggar (I never asked in the first place), an inconsiderate asshole (and other adjacent phrases), and had people dog on my own art because, for some reason, they thought being a worse artist than someone else required me to be grateful.
Other people telling me how to feel is why I just don't let anyone draw my stuff at all now.
I tried getting over this by letting someone do something that was only ever shown to people in private. I figured I could just clean it up on my own time, but our styles are too different for me to work with, and the work involved in correcting things made me just want to redraw it instead. Some of those things were specific to their own style as well, and warping one style into another requires quite a lot of effort.
The second time, money was involved; someone paid a lot to get gift art for me and the artist who did it has incredibly limited commission slots. Too much actual financial investment was involved for me to just say no, and the art was made by a generally skilled artist so I ignored the imperfections. There was only one thing that I could not ignore no matter how much I told myself to, and they did oblige me when I requested a change. I appreciated their understanding, but the whole thing still made me feel sick.
In the famous words of DBZA Cell: "It doesn't bother me. It doesn't bother me. It bothers me. It bothers me a lot."
I have some very old stress tied up with this, and it compounds my frustration with my own lack of skill. I'm also terrified that a more popular and prolific artist drawing my characters will color others' impression of how they're supposed to look.
When it's a comfort character, that paranoia becomes much more intense.
I think there is an expectation that we should appreciate different interpretations of characters and ideas, that we should embrace stylistic differences, and that 'everything is canon', and I have mostly masked my own disagreement with that for the sake of fitting into a community I otherwise enjoy.
I also do care about people and their feelings so I never feel good about requesting they take down art they worked hard on. I know a few people from my community follow me on here, so I'm making this post to hopefully dissuade people from putting in time and effort in the future that will be requested to take down. I won't ask anyone who's done something for me in the past to remove their work, some things have been up for months and I've already said yes, and I think going back on that now would be unfair, but please don't do it in the future.
There are plenty of people who greatly appreciate having their characters drawn, and I would rather people spend their skill and effort on those people.