I'm sure I've been that guy before at my gym, but it has nothing to do with the people to the left or right of me. Counterintuitively, its actually probably the least creepy choice at my gym to take that middle treadmill.

At my gym almost all the cardio machines (including treadmills and stationary bikes) are all in long rows facing one another...except 3 treadmills on one aisle, one stair machine on another aisle, and one stationary bike on yet another aisle. So unless you actually like staring at someone else 5 feet in front of you, with both of you exhaling from exertion right each other's faces, those 5 machine are you only choices. Any time one of those non-facing-someone-else-machines is open, I'll choose those first. I don't think I'm the only one that's noticed this because those seem to be the most occupied machines. I'd much rather be standing next to someone with both of us facing the same direction away from one another rather than having to stare and breath on each other as we work out.