Why didn't anyone tell me about this?
No matter how careful you are with your hardwood floors (have family and guests remove shoes, clean them with regularity, try not to slide things), the polyurethane protection that's below your feet, and on top of your floor, is designed to take the abuse so that the expensive part, your hardwood itself, doesn't have to and can last a lifetime. But polyurethane, much like sunscreen on a sunny beach, doesn't last forever with one application. Sunscreen wears off with water, activity, and play, and so does your polyurethane. As you and your family go about living your lives, this protective layer gets thinner and thinner. If this protective layer deteriorates for too long, eventually it will begin to expose bare wood, and you will start to see graying areas in the most heavily trafficked areas. At this point, no amount of cleaning can help and, in fact, can accelerate the problem because your wood is now exposed to chemicals, water, and has no scratch protection. Eventually, the floor you once loved so much is just too far damaged, and it will need to be completely resanded down to raw wood again.