I was listening the Don Carson speak on the transcendent glory of God recently. He was comparing the true God with the made up gods and religions of the lost. Those gods had deficiencies of one kind or another that humans could meet with sacrifices, good works or rites of one sort or another and then their god would reward them with something. This pretty much sums up most false religions it seems. As Carson put it, you scratch my back and I will scratch yours; a kind of tit for tat arrangement.
But as I was considering the transcendent glory of our God I began to see why God can only deal with us by grace if he is going to do us good. The thought came to me that the "bigness" of God necessitates grace. Because he is so glorious, he is self sufficient, he is completely satisfied in himself and therefore completely content with us or without us that he has no need of anything that we can do for him. This is the essence of perfection it seems to me. The aforementioned gods might be powerful but they can't be perfect by a long shot. But that which is perfect can only give since it has need of nothing. There is nothing man can do to "scratch his back". So if salvation or any blessings must come by us making him our debtor we are in trouble.
It is because God is so great that he can only be gracious. The only thing that can adequately display his perfection is not us giving him things but him sharing his glory with us. His "bigness" necessitates his graciousness.