Charles H. Spurgeon on Psalms 40:8

I delight to do thy will, O my God. Our blessed Lord alone could completely do the will of God. The law is too broad for such poor creatures as we are to hope to fulfil it to the uttermost: but Jesus not only did the Father's will, but found a delight therein; from old eternity he had desired the work set before him; in his human life he was straitened till he reached the baptism of agony in which he magnified the law, and even in Gethsemane itself he chose the Father's will, and set aside his own. Herein is the essence of obedience, namely, in the soul's cheerful devotion to God: and our Lord's obedience, which is our righteousness, is in no measure lacking in this eminent quality. Notwithstanding his measureless griefs, our Lord found delight in his work, and for "the joy that was set before him he endured the cross, despising the shame." Yea, thy law is within my heart. No outward, formal devotion was rendered by Christ; his heart was in his work, holiness was his element, the Father's will his meat and drink. We must each of us be like our Lord in this, or we shall lack the evidence of being his disciples. Where there is no heart work, no pleasure, no delight in God's law, there can be no acceptance. Let the devout reader adore the Saviour for the spontaneous and hearty manner in which he undertook the great work of our salvation.