Why do people so this?

I can't figure it out. Why do people underline and make notes in a unviersity library book? What is especially annoying is that the person knows it's a library books, knows they shouldn't put pen marks in a library book, but goes ahead and does it anyway. Why? Why? The arrogance of these people.

I must also admit that sometimes notes in books are funny. I am now looking at a table of contents for a translation of Aristotle's Physics, and written next to 'COMMENTARY' is 'or, obscurum per obscurius'.