The one supreme, unchangeable rule of love, which is a law to all intelligent beings of all worlds and will be a law to all eternity, is this, viz., that God alone is to be loved for Himself, and that all other beings only in Him and for Him. Whatever intelligent creature lives not under this rule of love is so far fallen from the order of his creation, and is, till he returns to this eternal law of love, an apostate from God and incapable of the Kingdom of Heaven.
Now, if God alone is to be loved for Himself, then no creature is to be loved for itself; and so all self-love in every creature is absolutely condemned. And if all created beings are only to be loved in and for God, then my neighbour is to be loved as I love myself, and I am only to love myself as I love my neighbour or any other created being, that is, only in and for God.
William Law: The Spirit of Prayer.