The Watchmaker's Ironic Repair

The puzzle

A customer visited an old watchmaker's shop. Proudly, he presented a broken pocket watch and asked, 'I'll pay any price, just make it the most accurate watch in the world.' The craftsman took it without looking up and replied, 'Very well. But I will make it run faster than you can ever outrun in your life. Do you agree?' Later, the customer received the watch, left the shop, and broke down in tears. Why?