It was the Romans who tagged to the broken mirror a sign of seven years bad luck. The length of the prescribed misfortune came from the ancient Roman belief that it took seven years for life to renew itself. If the persons looking into the mirror were not of good health, their image would break the mirror and the run of bad luck would continue for the period of seven years, at the end of which their life would be renewed, their body would be physically rejuvenated, and the curse would be ended.
In old times, mirrors were not cheap and they were low quality and easily defected. In order to avoid negligence it was told that breaking a mirror bring seven years of bad luck. That was simple scare tactic.
A variety of remedies are available to break the spell of misfortune. The Romans are also responsible for little known measure for avoiding the curse useful for anyone who breaks a mirror. The luckless who accidentally break a mirror, and does not wish seven years of ill-lack, must take all the pieces of the mirror and bury them in the moonlight, or take all pieces and throw them into running water, or pound the broken mirror into tiny pieces so that none of them can reflect anything ever again.