They had both lost many people in their lives - each getting wounded in the process. When they team up to fight for the other's right, they realize their potential together. Yet, he isn't able to bring himself to love her until she leaves for good.

Zach talks of proposing to all three of Lucy's parents in chapter 41, but the author never shows that scene in Impossible. So, brought to you, by yours truely, here is my version of that scene.