To explain our faith we need to reach back in the scriptures,
For he chose us in him before the creation of the world to be holy and blameless in his sight. (Eph. 1:4)
Before God ever created the first atom of the universe, He knew his entire plan. We see this most fully in the many ways that the Old Testament foreshadows the events in the New Testament. God prepares, not days or weeks ahead, but centuries ahead. He prepares individuals for their unique missions.
Secondly, we must remember the commandment,
Honor your father and your mother, so that you may live long in the land the LORD your God is giving you.(Ex. 20:12)
We believe that in all of human history God became incarnate one time, in Jesus Christ. The life, death, and resurrection of Jesus were unique events. If we can agree that the events of Jesus were unique, should we not also agree that the role of Mary, as the one through whom he took on our humanity, was unique? And worthy of remembrance and celebration ?
And if we believe that we are brothers and sisters in and of Jesus and she is his mother, how could we possible say that she is not our mother?
And are we not commanded to honor our mother?
Today we celebrate God bringing his plan for Mary to its logical conclusion. Her role in history was unique and so the completion of that role was unique. What son would not do likewise for his mother?