Very good question. This has plagued me for years, but I think I have some sort of answer.
It also has something to do with the Felix Culpa thing … the idea that Adam’s sin was a “happy fault” because it won for us the savior. This leads to more questions possibly. But original sin seems to be necessary for an Incarnate Savior, which, of course, shows forth the glory of God. Sin is a darkness that makes the goodness of God even brighter, for it shows God’s power to bring back creatures that appeared to be lost to Him. I think that’s why the majority of humanity was born into sin … what do you think?
I agree also that it has something to do with faults. God knew the way he was to bring us back to Him after Adam and Eve. He knew He would make us more than we ever could realize by adopting us as Sons and Daughters. He also wanted to make it as Glorious as possible.
A really good story is one where someone triumphs over adversity or wins against the odds. So it is with God. When we persevere over adversity, (pain, evil, sin, etc) it gives God’s works in us so much more Glory than if we all just naturally wait for salvation. We work with God’s Grace to bring about a triumph that is more magnificent than could have been.
It is wonderful that God chose such a vessel as Mary is to be the source of our Incarnate Redeemer, but it is also wonderful that we can be in the same family by His adoption of us and our triumph over adversity.