I'm not legalistic at all about this, but I think it is undeniable that ad orientem makes more sense, given what is happening in the Mass.
If the prayers at Mass are directed to God (obviously prayers aren't directed towards the congregation!), then clearly it is more "natural" to face God in the Tabernacle on the altar while saying the prayers, and offering the sacrifice. That's why the Church for most of her history in most places has done it this way (just as we face the Tabernacle in eucharistic adoration). It doesn't come out of nowhere. It has a "primacy" of past history and liturgical tradition.
Like I said, I'm not legalistic or pharisaical about this; nor am I regarding communion on the tongue at an altar rail (that I myself do every week). I've noted several times in public posts how reception of the Holy Eucharist in the early Church was standing and in the hand for 6-9 centuries, depending on location. But I think arguments can be made in defense of this, without putting down the alternative.
Protestant pastors much more so "represent God to the people" because they don't have the Eucharist and are not offering the Sacrifice of the Mass, so that preaching is front and center. Those things make a huge difference. The Catholic priest is offering with the people, and an offering is towards an altar: analogous to the Old Testament priesthood that was continued in the New Testament by Christ being our high priest, and each Catholic priest being in persona Christi and alter Christus.
A priest can still offer without facing an altar (and a Mass where that happens is just as valid), but it clearly makes more sense and continues the priestly tradition since Moses, to face the altar; offer on the altar, etc. The high priest used to face the ark of the covenant, where God was specially present; now we have Jesus present Body, Blood, Soul, and Divinity, so there is more reason to face the altar to make the offering than there was even in the old covenant with bulls and sheep and other offerings.
Facing the congregation thing is much more consonant with Protestant theology and tendency against sacramentalism (even by their own self-understanding). Someone said it was more consistent with the New Testament priesthood; I countered by saying that it fits in more so with Protestant pastors, who aren't priests at all.
The priest and Jesus Christ are simultaneously offering (along with all present) because the Mass is a re-presentation of the historic crucifixion: it's made present. The Mass is a sacrificial offering, which is priest and congregation together (not contradistinguished from each other, insofar as the offering is concerned: symbolized by one facing the other). Together they are offering to God, as opposed to the priest merely talking to the people, or bringing God to them (that applies much more to the Liturgy of the Word and homily [i.e., the first part of the Mass] ). That's why the liberal "dialogue" thing applied to the Mass (i.e., after the readings and homily) was beside the point from the get-go.
[see further discussion on my Facebook page]
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