Why do we say "People who live in glass houses shouldn't throw stones"?

Meaning:

Don't be overly critical of others when you yourself could be criticized for the same sort of things.

Background:

The oldest known written use of this expression is in Geoffrey Chaucer's Troilus and Criseyde (c1385)

"Who that hath an hed of verre, Fro cast of stones war hym in the werre!"

About 200 years later in 1651, Welsh poet, orator and priest George Herbert records a more modern version: "Whose house is of glass, must not throw stones at another."

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