I was doing a relaxed longish training run when a hear this heavy breather getting closer and closer behind me. I felt my automatic response was to pick up my speed to make it more difficult for him to pass. I even might have a little.
By his breathing, I knew he was pushing it pretty hard, and that I was a better conditioned and faster runner. I had to concentrate on maintaining my training speed. Then after he passed I still had to fight off the urge to close back on him. I didn't but it took restrain not too.
After running so many races there its is just something you are conditioned to do. Hearing the footsteps and breathing closing in on you makes you feel competitive.