"Thy will be done on earth as it is in heaven..."

You may say, “Well, did God not endow us with a free will?” I reply: To be sure, he gave you a free will. But why do you want to make it your own will? Why not let it remain free? If you do with it whatever you will, it is not a free will, but your own will. God did not give you or anyone else a will of your own. Your own will comes from the devil and from Adam, who transformed the free will received from God into his own. A free will does not want its own way, but looks only to God’s will for direction. By so doing it then also remains free, untrammeled and unshackled.

In this petition you will notice that God bids us to pray against ourselves. In that way he teaches us that we have no greater enemy than ourself. You see, our will is the most formidable element in us, and against it we must pray, “O Father, do not let me get to the point where my will is done. Break my will; resist it. No matter what happens let my life be governed not by my will, but by yours. As no one’s own will prevails in heaven so may it also be here on earth.” Such a petition or its fulfilment is indeed very painful to our human nature, for our own will is the greatest and most deep rooted evil in us, and nothing is dearer to us than our own will.

Therefore, we are asking for nothing else in this petition than the cross, torment, adversity, and sufferings of every kind, since these serve the destruction of our will. If these self-willed people really thought about this and noted that they are praying against their own will, they would turn against this petition or even be frightened by it.

-- Martin Luther, "An Exposition of the Lord's Prayer for Simple Laymen," LW 42: 48-49