Responding to the dead child comment: that's what common paperbacks are for. Don't don't mess up a nice hardcover or anything rare.
But objectively my reaction is wrong. Books are not mystical objects to be revered. They are objects to be used. Nearly all books end up in a landfill or recycled eventually. What does it actually matter if they end up there covered in annotations and filled with dog eared pages?
Books you have borrowed? Absolutely do not write in them or dogear pages. Books you intend to share with others? Generally the same. Rare books or valuable books? Of course. Normal books you got on Amazon or from your local bookstore that you use only for your personal enjoyment? Use them how you want.
I'm not being black and white like that. Some books "are to be read, annotated, lived with and lent out" others should be treated with more care.
For an extreme example: if somehow you come to possess the Book of Kells, don't go scribbling your brain farts all over it. You're a modern person who can easily buy paper or a notebook to hold such things.