I've never been one to deface books by highlighting, underlining, or taking notes in the margins, though I know lots of people who do. However, I can only laugh when I borrow a book from the library and read some of the utterly pointless and moronic notes the previous borrowers took it upon themselves to leave for the rest of the world to see.
Right now, I'm writing a speech on Albert Einstein, and so I borrowed a book with excerpts from his correspondence. One paragraph reads:
I have to tell you frankly that I do not approve of parents exerting influence on decisions of their children that will determine the shapes of the children's lives. Such problems one must solve for oneself.
Penciled into the margin are the words "personal problem
Another paragraph reads:
Something there is that can refresh and revivify older people: joy in the activities of the younger generation--a joy, to be sure, that is clouded by dark forebodings in these unsettled times.
In addition to this passage being underlined, written in the margin is the word "positive
What are people thinking when they write notes like this? They add nothing to the book or its meaning, they display little or no insight regarding the passages they're attempting to annotate, and they're distracting to the next person who decides to read it, though they are good for a quick chuckle.
There are a great many activities and observations that renew my faith in the potential of humanity--borrowing books from the library is not one of them.