If God's desire was to be fulfilled, that is reconciliation with man, it WAS a necessity. If God desired not to show mercy upon human beings and be reconciled to them, then no it wasn't. We could have paid our own debt and would spend eternity in Hell. It was God's desire that made it a necessity - not some external force. The dichotomy is false.
I give you a gift because I desire to give you a gift and you are telling me that I am obligated to give you a gift!?