Picture a piece of common ground. This common ground is used by four villagers who each keep a cow on it. They milk their cow to meet their own demand and sell any surplus in the village to buy luxuries they might want.
One day, a villager realises that if he bought a second cow he would have far more milk to sell and so would be able to buy more for himself and his family. The other villagers see this and so also buy an extra cow to graze on the common land. All this milk on the market causes the price to fall and suddenly our first villager needs to now add a third cow to get the extra money he wanted. The others feel compelled to do the same. Over time, more and more cows get added until the land is destroyed and can support none.