Mercygate made a very good point in citing Ratzinger’s essays. I think that this gets at the answer.
Remember that in the Canon of the Mass - that is, the part between the Sanctus and the Great Amen - the Sacrifice of Christ is being re-presented to the Father. It is this Sacrifice by which we were freed from our sins. Before the Sacrifice of Christ, humanity was in a servile relationship to God, whereas after the Sacrifice, the relationship is familial - Father to children.
And so as the Canon begins and we praise God with the Angels singing “Holy, Holy, Holy,” we symbolically behave put outselves into that original, servile relationship. We get on our knees as sinners before God and declare His holiness. Just as the people of Jerusalem blessed the Messiah with cries of Hosanna as He entered their midst to offer Himself, we do the same as Christ prepares to enter our midst and offer Himself.
The Sanctus then has two symbolic purposes - or at least two that I am aware of. First, it joins the cries before God of those in Heaven, “Holy Holy Holy,” with the cries of those on earth, “Hosanna in the highest,” to symbolize the meeting of Heaven and earth that is about to occur as the eternal, heavenly Sacrifice of Christ becomes present to us here on earth. Second, it both symbolically and also in a very real sense unites us with the sinful people of Israel as we praise the Messiah entering our midst who is to save us from our sins.
And so we kneel after the Sanctus. We kneel as Christ is made present to us just as all Creation knelt as the child Jesus entered the world at the Nativity. We also kneel before God Almighty, who is present on the altar, and whom we are - at least symbolically at the point in the Liturgy - sinful before.
Then comes the Great Amen. The Great Amen is the ultimate consummation of the Sacrifice. It is when Christ, acting through the priest, offers Himself to the Father, and we offer ourselves and all we have as well “through Him, with Him, and in Him, in the unity of the Holy Spirit.” The Amen is “great” because in it the faithful give their assent to the Sacrifice that has just occured, making it their own. With the Amen, they accept the Sacrifice which has been offered on their behalf and give assent to all of the prayers which have come to this point. They here agree to give themselves and all they have to the Father through Christ. Of course, the Sacrifice and prayers would be accepted by the Father even without the participation of the faithful, but with the great Amen each person accepts that Sacrifice individually.
And so after this, we rise. Symbolically, we are risen from our sins. The Sacrifice has been offered, and we are forgiven. We no longer are trapped in a servile relationship to God, but are now His children, and so we stand and call Him Our Father, and ask Him for our needs. We then exchange the sign of Peace, which symbolizes the communion we now have with each other by virtue of having been brought into the Communion of the Saints. I shake hands with my neighbor after the Great Amen to represent and on a deeper level to live out the fact that, thanks to the Sacrifice that has just occured, we are now brothers in God’s Communion, just as we say “Our Father” after the Sacrifice because this Sacrifice has made us His children.
We then say the Agnus Dei, which is a hymn of praise and thanksgiving to Christ for what He has done for us, for the Sacrifice He has offered. It is also a hymn of petition as we ask Him to continue to grant us mercy and peace.
And then we kneel again, but our kneeling this time is in adoration of the Lamb - even those in Heaven kneel before the Lamb. In the course of the Liturgy, with the Sacrifice having been made and our Communion having been expressed, with our Divine sonship having been lived in our prayer to Our Father, we now kneel as the saints in Heaven do, in adoration of the Lamb and in thanksgiving.
And as the saints in Heaven do, we then partake of Communion with God Himself.
