In most cases people apply for jewelry workshops with request of soldering the broken chain. There are several reasons for that.
1. The abrasive wear. In this case rings just chafe at each other. This can be seen in a hand-glass with high magnification or when rolling two ends of simple anchor chain over the eyes. The old chain will be rolling over irregularly as it were small sticks of random length. The worst wear can be found at the end of these sticks.
2. Production defect when the rings are poorly soldered. But fortunately such items are rarely come into retail network.
3. Chains of foreign manufacture. These goods are manufactured of thin hollow straws instead of high quality wire. They are well-seeming and have reasonable price due to light weight, but that particular items are mostly send to repairs.
4. Also the problem can be caused by mismatch of the chain and the jewelry worn weight. You should not wear pretty heavy cross with a thin chain due to the reason that soft metals as silver and gold can be chafed off.