When you are speaking to someone who knows you well and who is in tune with how you think, you might start to say something, and that person will complete … your sentence. If you are in relationship with that person, you find it to be a bit endearing, “Yes, we are one in what we are and in what we do.”
The exchange of greetings between the priest and the people at the beginning of Mass can be enjoyed in that context. In writing to Timothy in his second letter, Paul greets his friend, “The Lord be with your spirit.” Knowing Paul well and being in a good relationship with him, Timothy could have completed Paul’s sentence. Paul begins, “The Lord be …” and Timothy continues, “… with your spirit.”
As a priest, I like the feel of thinking that the people and I could say the same thing to each other. Since the Holy Spirit is at work in both of us, I could rightly speak to them, as Paul spoke to his congregations, “The Lord be with your spirit.” And they could rightly respond to me, as Paul spoke to Timothy, “The Lord be with your spirit.” But at Mass, instead of saying it twice, we say it just once, splitting it in two parts, each of us speaking half of it. It is like we are so connected and are so joined in what we are about to do that I begin and they complete … the sentence, “The Lord be [with you] … [and] with your spirit.”
From the very first words spoken at Mass, and at very key places in the Mass when something real important is going to be done, the priest and the people establish clearly that they are dependent on each other, as together, and together with Christ, they offer the sacrifice of the Mass.
This is how it will sound if the dialogue is chanted: And with your spirit.
With Open Doors 
I was hoping for something inspiring today, Fr. Rob. How beautiful to think of this connection!