The cause and need of this commandment is that God well knows that the world is evil, and that this life has much unhappiness; therefore He has placed this and the other commandments between the good and the evil. Now, as there are many assaults upon all commandments, so it happens also in this commandment that we must live among many people who do us harm, so that we have cause to be hostile to them…
Thus by the devil’s incitement you will get many enemies who cannot bear to see you have any good, either bodily or spiritual. When we see such people, our hearts, in turn, would rage and bleed and take vengeance. Then there arise cursing and blows, from which follow finally misery and murder. Here, now, God like a kind father steps in ahead of us, interposes and wishes to have the quarrel settled, that no misfortune come of it, nor one destroy another. And briefly, He would hereby protect, set free, and keep in peace every one against the crime and violence of every one else; and would have this commandment placed as a wall, fortress, and refuge about our neighbor, that we do him no hurt nor harm in his body.
Dr. Martin Luther, Large Catechism, I.183, 185
Locus Communis 