This point can also be illustrated by comparisons. (I make comparisons for the sake of ordinary people, who see more in a comparison than they do in a deductive analysis based on the Word and on reason.)
When any citizens or subjects obey the commands and orders of their king, they are united to him. If they endure oppressive circumstances for him, they are more deeply united to him. If they suffer death for him, as happens in battles and wars, they are still more deeply united to him.
In the same way, doing the other person’s will is how a friend is united to a friend, a child to a parent, or a servant to the head of the household. If the friend, child, and servant defend their superiors against enemies they are more deeply united to them. If they fight for their superiors’ honor they are even more deeply united to them.
Take for example a young man and the young woman he hopes to marry. When he confronts people who are destroying her reputation, surely he becomes more united to her. What about when he is injured fighting a rival? It is a law inscribed on nature that under these circumstances the couple will become more deeply united.
The Lord says, “I am the good shepherd. The good shepherd lays down his life for the sheep. For this reason the Father loves me” (John 10:11, 17).
from True Christianty, Volume 1, Section 131