A chain can stretch for a number of reasons. The most common reason is simply wear and tear. As a chain is used, the links rub against each other and slowly start to deform. This causes the overall length of the chain to increase slightly. Another common cause of chain stretching is incorrect tension. If a chain is too loose, it will sag and deform more easily under load. This will also cause an increase in overall length. Finally, some chains are simply made from inferior materials that are more prone to stretching. Regardless of the cause, a stretched chain will eventually need to be replaced.