John Gill Commentary Psalms 40:8

"I delight to do thy will, O my God; Yea, thy law is within my heart." — Psalms 40:8 (ASV)

I delight to do your will, O my God
This he came down from heaven to do, and this he did do, by preaching the Gospel, and working miracles; and above all by obtaining eternal redemption for his people, which he effected by fulfilling the law, becoming a sacrifice, and suffering and dying in their room; all which were the will of God, and grateful to him, and in doing which Christ took the utmost delight and pleasure, (John 4:34) (Luke 12:50) ;

yes, your law [is] within my heart ;
either the whole moral law, under which he was, as man, and the surety of his people; and which was written upon his heart, and which he perfectly obeyed; or that particular law, injunction, and command laid upon him by his Father, to offer himself a sacrifice, and lay down his life for men; which he agreed to, had it in his mind, his heart was set upon it, and he cheerfully complied with it, (John 10:18) (14:31) .