There are always great discussions about this ancient custom prevailing over how Mass ought to be celebrated. Recently Cardinal Sarah suggested that priests return to the practice of celebrating Mass facing east, as the Extraordinary Form is traditionally celebrated. I grew up with Mass celebrated like this, and when I returned to the sacraments in 1972, a few local parishes had not turned their altars around yet. St. Agnes in St. Paul, a parish I frequented, still celebrates Mass ad orientem. I actually prefer it - but I accept the other - facing the people. (I prefer kneeling for communion on the tongue as well, but I accept the other.)
That said, this morning's first reading at Mass seems to establish the reasoning behind the ancient practice quite beautifully - actually, quite gloriously.
The angel led me to the gate which faces the east,and there I saw the glory of the God of Israelcoming from the east.I heard a sound like the roaring of many waters,and the earth shone with his glory.The vision was like that which I had seenwhen he came to destroy the city,and like that which I had seen by the river Chebar.I fell prone as the glory of the LORD entered the templeby way of the gate which faces the east,but spirit lifted me up and brought me to the inner court.And I saw that the temple was filled with the glory of the LORD.Then I heard someone speaking to me from the temple,while the man stood beside me.The voice said to me: Son of man, this is where my throne shall be,this is where I will set the soles of my feet;here I will dwell among the children of Israel forever. - Ez 43:1-7AB
Hebrew illumination of Ezekiel's vision.