C. Why should the priest turn round to the people when he is engaged in so solemn an act of communion with Almighty God?
P. To assure them continually of his good will toward them, to remind them that they are parties with himself in the great act he is performing, and to keep up their attention; even as our blessed Lord Himself broke off three several times from His prayer in the garden in order to sustain the fainting hearts of His Apostles: and hence the Church would have us remember that our life on earth is divided between the duties of devotion and charity, for on those "two great commandments hang all the Law and the Prophets." But you will find that when the priest has once entered upon the more solemn parts of the Mass, he no longer salutes the people by turning toward them.
(Source: The Glories of the Catholic Church The Catholic Christian Instructed in Defence of His Faith by Richard Challoner, et. al.)