And I don't even have to go into how rude it is to read over another person's shoulder. It just is. You are not to position yourself so closely to a stranger as to make them uncomfortable. It defies the laws of common courtesy. (And you all know how big of a proponent of common courtesy I am.)
Let me just also say that reading over someones shoulder when you have B.O. is completely UNACCEPTABLE. If you sense for even one second that you have B.O. (which you should totally be aware of, because if not, I feel REALLY bad for you) or smell like moth balls or have bad breath, DO NOT, I repeat, DO NOT ever read over someones shoulder. Yesterday, I got on the train and began my usual routine of reading the papers. At the next stop, two 20-something fellows boarded the train and instantly, the entire car filled with the scent of B.O. - B.O. that had been marinating in a gym locker for 4 years. And of course, these two lovely gentlemen sat down right next to me. I tried my hardest to ignore the scent, but it became literally unbearable when the fellow closest to me (covertly - except NOT) scooted over to try to catch a glimpse of my paper. AWFUL times a MILLION. No joke. Serious as serious can be. I could feel the vomit rising in my throat.
I can't forget to mention the fact that if someone is reading over your shoulder and you're aware of it, you are suddenly in the position of questioning whether to turn the page when you're finished reading it or whether to turn to the next page when you notice that the over-the-shoulder-reader is finished reading. This is like the BEST case scenario. This is what would happen if you're the nicest person in the world, which clearly, I am not. But even so, the over-the-shoulder-reading causes problems. Period.
On a personal note, I do not react well in this particular situation. When I catch someone reading over my shoulder, I tend to make a big deal of turning away and huffing and puffing. I will probably make you feel bad about doing it - which you should. So stop.