I've always and I'm not sure why, but when I sit in a classroom I go to the aisle seat, then move one in. Keep in mind most of my classrooms are large auditoriums of 300 seats or more. Why the second one in?
I can't decide if I want to believe that I am welcoming someone to sit there, or If I'm deterring people from sitting there. A person's natural instincts is to find a seat with no one on either side of them (unless of course you're with friends). So for those occasions where I'm flying solo, why not get a seat with no one on either side?
I've met several people each semester just from leaving that one seat open. But at the same time I've had plenty of courses come and go with that empty seat remaining empty.
Another funny thing to me is when you start to notice people sitting in the same spot for every class. Every week comes around and you can find that same complete stranger to you in the front row on the right aisle or in a small class, in the back corner over you're right shoulder. I think I'm going to start messing with these people in larger classrooms. Sit right in front of them as if I'm bugging them in a movie theater. Or go up to somebody in their "normal" seat and inform them that they have taken my seat.