Why is breaking glass good luck?
Breaking glass is not good luck- it is bad luck.
If you have bad luck coming your way, breaking glass will,
according to superstition, eliminate it. Breaking glass, without
any (bad luck) precursor, will supposedly manifest itself in only
bad luck.
Speaking in more detail, according to superstition, when one has
bad luck coming his or her way, breaking a mirror works as a shield
to bad luck. A mirror is a reflective device. When one breaks a
mirror, it will reflect the bad luck of a broken mirror. But, when
already in the presence of bad luck, it reflects the bad luck in
the bad-luck dimension. Bad luck in the bad luck dimension is good
luck, which we all obstinately have.