Injuries
In an ideal runner's world, every step of every mile would be 100% pain-free. No aches, no twinges, no lingering soreness from yesterday's workout. The reality is that many runners constantly deal with a slight (or not so slight) disturbance - a tender foot, a tight hamstring, a whiny knee.
While these nagging issues often aren't serious enough to require a time-out, they are annoying, especially when they don't let you fully enjoy your time on the roads.