."It is written (Mishlei 27:19): “As a face reflects its face in water, so does one’s heart to another.”
Why in water? Why not in a mirror? Because to see one’s own face in the water, one has to bend down (a sign of humility). Not so with a mirror. With a mirror, one can remain upright and erect (symbolic of arrogance)."
Rabbi Simcha Bunam of Pshis’cha