Habit --> Automatic response.
I'm not sure if this situation is normal, but according to a book called "The Organized Mind" by Daniel J. Levitin,
"The inner dialogue is generated by the planning centers of your brain in the prefrontal cortex, and the questions are being answered by other parts of your brain that possess the information."
From the neuroscience point of view, I guess it's normal after all. Eventually, I had become a sage of my own making. The discussion could encompass a wide range of topics, from the meaning of life to the "what ifs" of situations, the latter being one of my favorites. Oh, don't get me started on the interpretation of movie/music from different point of views.
So, if you see me from a distant, that I was alone and quiet at a corner of a restaurant, assume not I was silent. I could be trying to solve models of equations to create a viable cure for cancer in my head.