I don't highlight or write (with pen), but I do, to my BFs horror, occasionally dog ear. Like Britwife, I think teachers and librarians were so strict about writing in books, I've never been able to get past it. I only dog ear if I own the book and I'll always go back and fix them when I'm finished. I can't stand to buy a highlighted text book as I'll never read between the highlights.
As for loaning out books, I never used to do it, but then I kind of thought of it as "paying it forward". You know, I find a book I really enjoy, I should pass it on to someone else to enjoy and hope they pass it on to someone else. My good friend did this to me with a book about a group of 4 girlfriends and I thought that was kind of cool.