They were the one. The chosen one.
Who cares! They were the one. They had to do this.
No point in giving a reason, for this was destiny, put in place by the spirits above who pulled all the strings.
But wait… why were they the chosen one? They didn’t have any special qualities, any differing talents, or—once they thought about it—any personality of any kind.
They’re the chosen one.
And that’s the author’s favorite trope.