Chain wear, otherwise known as chain stretch, is caused simply by dirt. There are minute gaps between the link sides so that the chain can flow freely, and dirt can get between within these gaps and cause damage. Make sure you clean the chain often; every time you go off-road in wet conditions with a mountain bike or hybrid. We'd recommend doing this in dry conditions as well, due to dust particles. If you're commuting on your bike, clean your chain at least once a week, or more often if your doing heavy mileage.