Q. Why do we not wear leather shoes on Yom Kippur?
A. Leather shoes, which are solid and comfortable, are symbolic of physical pleasure. This is one of several marks of physical pleasure forbidden on Yom Kippur, when the Torah tells us to afflict our souls (Lev. 23:32).
Obviously, if we fail to afflict ourselves we will be too comfortable to do anything about our spiritual and ethical selves.
A well known interpretation reminds us that when Moses came to the Burning Bush (Exodus 3) he was told to remove his shoes because it was holy ground: as a sanctuary in time Yom Kippur is also holy ground. Hence the appropriate footwear for the day is canvas, rubber, plastic or other non-leather shoes.