I can't bear anything with animal cruelty/suffering in it. I've given up on many books because of that though occasionally if the book is good enough, and it isn't a recurring theme, I'll just skip that part and continue.
I also really can't bear to read anything relating to witch hunting. It makes me feel so ill to think about it.
I'm also not a fan of romance for the sake of romance itself. If there's some pressing reason for them to be together or if they've been through a lot as a couple then I don't mind but otherwise I tend to avoid romance.
No judgement. Each to their own but I'm curious to know which subjects/genres you make a habit of avoiding?