Because I came down from heaven (cf. John 3:13), not that I might do my own will, but the will of him that sent me (see John 5:19, John 5:30, notes). The practical, ethical force of this statement is to shape and defend the previous assurance.
Christ's gracious reception and benediction is in willing harmony with, and not in opposition to, the Father's heart. There is no schism between the Father and Son. A separate will in and of itself assigned to the Son is not inconceivable, nay, it is imperatively necessary to posit, or we should lose all distinctions whatever between the Father and Son, between God and Christ.
But the very separateness of the wills gives the greater significance to their moral oneness. "Not my will, but thine be done," "Not as I will, but as thou wilt," involve submission, voluntary surrender, to the Father's will; but here the Lord insists on absolute harmony and free cooperation.
The bare idea of the Incarnation suggests the conditions of freedom which might conceivably issue in divarication of interest and aim. Christ declares that the Divine commission of his humanity is the spontaneous and free, but perfect, coincidence of his will with the Father's.
Christ's embodiment of the Father's will, and coordination with it, make all his attractiveness to the human soul. His healing, feeding, and satisfying powers become a revelation of the Father's heart.
If he will not cast out the coming ones, it is because he came down out of £ heaven to fulfil the Father's will (see further, John 6:44, John 6:45), to explain the world wide hunger, to meet and execute the will of the Father.
The frequent assertions by our Lord in this discourse (and in John 3:13) of his descent from heaven as One charged with a full knowledge of the Divine will, implies that the Lord was conscious of pre-existence in the very bosom of God.
This was language which, with more of the same import, led St. Jn to the overwhelming conclusion that the Jesus whom he knew in the flesh was the Only Begotten of the Father—was the Logos made flesh.