Why Did Jesus Really Have to Die For Us?

Why did Jesus really have to die for us?  Many Catholics with a PhD in theology would not be able to say more than a 9 year old:  “To die for our sins.”  We look to St. Catherine of Siena for an explanation that is simple enough for a child, but complex enough to [hopefully] satisfy the 12 families who gathered in Louisiana to hear about this central tenant of Our Faith.