
It's always a two-way street.  God gave her the grace to avoid sin,
thus when she was visited by Gabriel, she gave her fiat, her total
acceptance of God's will.  This fiat summarizes why Catholics regard 
her as the highest of all humans, that God chose her and that she
accepted.  Knowing this in advance, we extrapolate that she was
neither stained by nor subject to original sin.

God did create us all miraculously free to choose or not choose to sin.
"Sufficient for the day is the evil thereof and the grace of God to
command it."  This amount of grace was precisely determined by God
to be the amount required to do what God asked of her.  The grace
given to each of us is also enough, but we do not always choose
to accept it.  We also believe Jesus was fully human and never
sinned.  

God could have created a much better person than myself, one who
always chose the right thing, yet he created me instead, despite
my flaws.  He proves he loves me as I am, continually drawing me
towards perfection.  For whatever purpose he has for me, he has
confidence that I will accomplish it.  If I ask God to repeat his
miraculous creation of the mother of his son, where will that leave me?