As a non-Catholic, could someone please explain the reasoning behind this?
The Immaculate Conception refers to the fact that through a singular Grace given by God, Mary was preserved from all stain of Original sin at the first moment of her existence at her conception in the womb of her mother, St. Anne.
The Virgin Birth refers to the conception of Jesus by the Holy Spirit with Mary's consent at the Annunciation and His subsequent Birth at Christmas.
Some people confuse these two events of Salvation History.
The Immaculate Conception of Mary was a miracle worked by God being necessary for the 2nd Person of the Blessed Trinity to become Man. From Eternity, God created, chose, and prepared Mary to be the Mother of His Divine Son, and through Him, of all people redeemed by His Cross. It was fitting that she who was to be His Mother, should be free of the stain of all sin, or from the other side of the coin, "full of Grace". This Grace, as are all Graces, is a result of Christ's Passion, and not separate from it.
This teaching has always been believed by the Church, in both the East and West, and it's feast day celebrated from the earliest times. It was not "invented" by the Pope, but rather proclaimed as dogma to clarify correct belief, as the Church's belief continually develops to deeper understanding. Indeed, theologians had been arguing about it's nuances for Centuries (which is their job), triggering the Pope's proclamation (which is his job).
Hope this short synopsis helps!
Could you explain the following to me?
You believe that Mary had to be sinless because she was to have a sinless Son of God
Did Anne need to be sinless to have a sinless daughter that would be sinless? Then did Mary's grandmother need to be sinless so her sinless daughter could have a sinless daugther that would have a sinless Son of God?
If God in a moment of Grace could cause Mary, who had a sinful mother AND a HUMAN sinful Father to be born sinless was he UNABLE to have His sinless Son born to a woman that bore the original sin of man?