Mirrors were not common in most households until about the 19th century. They were simply too expensive to make, with early versions just being large pieces of polished silver, and owning one was often considered to be a symbol of affluence. As such, they gained a sort of mystical association, and had numerous folk beliefs attributed to them by people who couldn’t believe that’s what their hair actually looked like. The most famous of these notions was that if you broke a mirror, then you would experience seven years of bad luck.
Why was this so? It’s not known, but may be due to the fact that mirrors were supposed to hold the secrets of the future. If you broke one, then you shattered your future, and were pretty much consigned to spending most of the next decade watching your sheep die off and getting accidentally locked in funeral caskets. Some people believed that the curse could be reversed by lighting seven candles and blowing them out at midnight, or by burying all the pieces of the broken mirror underground, but that was just silly superstition. If you broke a mirror, you were (quite literally) out of luck.