Why is it bad luck to break mirror?

The history of breaking mirrors resulting in bad luck comes from the idea that it typically used to cost 7 years' wages for the average person to buy a mirror (being that it was made from glass with real silver on the back). As time went by, the prices of mirrors dropped dramatically, but the superstition stuck.

That's really the case with most superstitions: they began as an observation about the way things used to be, and then became rule which no longer applies.

But breaking mirrors still isn't advised: those glass shards are sharp!