The belief that a broken mirror brings bad luck most likely has its origins in the simple fact that reflections of ourselves are uncanny and often unnerving (particularly on a "bad hair" day), so humans have long had bad associations with them. Take, for example, the Greek myth of Narcissus, or the idea that a crack in a mirror would somehow break its charm or trap one’s soul. It was the ancient Romans, however, who contributed the notion that a broken mirror would bring seven years of bad luck, since it was believed that only poor health would cause a mirror to crack, and the number seven was seen by the Romans as the number of years required to complete a full life-cycle of sickness and renewal. As a result, a broken mirror meant you were headed toward a death-spiral that might take seven years to pull yourself out of!
