I don't like writing in any books. I do, sometimes, because I'm too lazy to take notes on separate paper. But I always write in pencil in the (clearly false) hope that I might erase it. The point is that I could erase it, and then the book would return to its pristine condition.
I absolutely despise those people who tell you that annotating a book is an act of love and not defacement. They are lying to you and they have no souls.
But I can accept writing in your own book. I don't like it myself, but perhaps I love books more than most people (the wording here is ambiguous but both readings are accurate--I both love books more than I love most people and love books more than most people love books). Writing in a library book? That's downright antisocial. Clearly it distracts from the text that is there. Also, most of the people vile enough to write in library books are complete morons, so the annotations are not only distracting but also inane.
What kind of entitled miscreant would ever think to write in a library book? They should be first against the wall when the revolution comes.