Luther's instruction on the 2nd petition of the Lord's Prayer "Thy Kingdom Come:"
...From this you perceive that we pray here not for a crust of bread or a temporal, perishable good, but for an eternal inestimable treasure and everything that God Himself possesses; which is far too great for any human heart to think of desiring if He had not Himself commanded us to pray for the same. But because He is God, He also claims the honor of giving much more and more abundantly than any one can comprehend,-like an eternal, unfailing fountain, which, the more it pours forth and overflows, the more it continues to give,-and He desires nothing more earnestly of us than that we ask much and great things of Him, and again is angry if we do not ask and pray confidently.
For just as when the richest and most mighty emperor would bid a poor beggar ask whatever he might desire, and were ready to give great imperial presents, and the fool would beg only for a dish of gruel, he would be rightly considered a rogue and a scoundrel, who treated the command of his imperial majesty as a jest and sport, and was not worthy of coming into his presence: so also it is a great reproach and dishonor to God if we, to whom He offers and pledges so many unspeakable treasures, despise the same, or have not the confidence to receive them, but scarcely venture to pray for a piece of bread.
(Large Catechism, 2nd petition of the Lord's Prayer)