Aha! I've got it!

I've puzzled over why I generally (but not always) am fascinated by animals turning into humans, but the reverse repulses and horrifies me. I've figured it out. However, it will probably sound a little arrogant/vain/etc.

For an animal to become a human, they're progressing to a higher state of being and are able to do more things. It's intriguing and fascinating to see how they handle it.

But for a human to become an animal, they're regressing to a lower state of being and are able to do less things. And to me, that's a horrifying and skin-crawling proposition that I don't even like to think about.

As for the times when I don't like animals turning into humans, I'm still trying to fully decide what motivates those feelings. Perhaps it's when I really don't care to see how that particular character handles it and feel they should remain always as they are. My Oreo character is one example; Charlie Barkin turning into a human on the All Dogs Go to Heaven series is another. (Although you know, I might have been more okay with that if Itchy had been turned too. From what I've heard about that episode, he wasn't.)