YOU KNOW WHAT REALLY UPSETS ME?

       Here is a situation on the interstate that has always baffled me. Here I am driving along on the right hand lane and  doing pretty much the speed limit. I look in my rear view mirror and here comes this vehicle gaining on me with lighting speed. Then, all of a sudden he slows down and maintains a position right at my "blind spot." He just stays there for a time. What is that all about? Is he afraid of coming up along me so I can see him out of shyness? Or what? I have never understood what happens there. How can you be driving along at a pretty good clip and then all of a sudden bingo! In the blind spot. Especially if that person has on his cruise control! Please let me know of the explanation!