The ba'alei mussar point out that the difference between a window and a mirror is but a little kesef-a little silver.
Kesef is derived from the word that means "to desire".
[psalm 84: nichsefa we-gam kal'ta nafshi...........My soul yearns ,also pines.....][nichsefa : compare the word kesef]
When one gives in to passions and desires, he sees only himself.
There is no room for G-d in this world.
[ form the book of Rabbi Zev Leff: Shemoneh Esrei]