After arriving to China I found it interesting that whenever I went over to a Chinese friends home they would always ask me to take off my shoes and put on sandals. I thought this was a bit bizarre at first and asked a Chinese friend why this was the case, she said "Well, it's simple really. Outside the ground is a dirty place. Why bring that dirt into your home." I realized "outside" is a dirty place, but in the States we are still okay from time to time to walk in the house with shoes on. My friends comment made more sense however, after I saw how Chinese people will treat the public walkways - as makeshift restrooms, where kids can pee on curbside and public buses, and even grown men can let one fly in the metro stations. It's true, and I've seen it on more than one occasion.
Moral of the story. I now have a very nice pair of sandals by the my door.