That's fine, too; I'm not suggesting that the only way to read books is to annotate them, just explaining why someone would want to. It's cool if that's not how you enjoy them, but making annotations isn't an "academic exercise" for me and it's not extra work, it's just a thing I do sometimes, especially if I'm particularly moved by a book. I read books for entertainment and because I find them informative. I revisit books with some frequency for various reasons, so the annotations are useful and entertaining.

It's not defacing a book if it's your book.