The best reason of all is because he cannot--he is dead. Well, the dead may be made alive, but the dead cannot make themselves alive, for the dead can do nothing. Besides, the new thing to be created has no being. The uncreated cannot create. "No," but you say, "that man can create." Yes, but can hell create heaven? Then sin may create grace. What! Will you tell me that fallen human nature that has come almost to a level with the beasts, is competent to rival God; that it can emulate the divinity in working as great a miracle, and in imparting as divine a life as even God himself can give? It cannot. Besides, it is a creation; we are created new in Christ Jesus. Let any man create a fly, and afterwards let him create a new heart in himself; until he has done the lesser he cannot do the greater.
Besides, no man will. If any man could convert himself, there is no man that would. If any man said he would, if that were true, he is already converted; for the will to be converted is in great part conversion. The will to love God, the desire to be in unison with Christ, is not to be found in any man who has not already been reconciled with God through the death of his Son. There may be a false desire, a desire grounded upon a misrepresentation of the truth; but a true desire after true salvation by the true Spirit is a certain indication that the salvation is already there in the germ and in the bud, and only needs time and grace to develop itself.
But it is certain that man neither can nor will, being on the one hand utterly impotent and dead, and on the other hand utterly depraved and unwilling; hating the change when he sees it in others, and most of all despising it in himself. Be certain, therefore, that God the Holy Spirit must begin, since none else can do so.