John of the Cross on Friendship

 6. Sometimes, again, there arises within these spiritual persons,
   whether they be speaking or performing spiritual actions, a certain
   vigour and bravado, through their having regard to persons who are
   present, and before these persons they display a certain kind of vain
   gratification. This also arises from luxury of spirit, after the manner
   wherein we here understand it, which is accompanied as a rule by
   complacency in the will.

   7. Some of these persons make friendships of a spiritual kind with
   others, which oftentimes arise from luxury and not from spirituality;
   this may be known to be the case when the remembrance of that
   friendship causes not the remembrance and love of God to grow, but
   occasions remorse of conscience. For, when the friendship is purely
   spiritual, the love of God grows with it; and the more the soul
   remembers it, the more it remembers the love of God, and the greater
   the desire it has for God; so that, as the one grows, the other grows
   also. For the spirit of God has this property, that it increases good
   by adding to it more good, inasmuch as there is likeness and conformity
   between them. But, when this love arises from the vice of sensuality
   aforementioned, it produces the contrary effects; for the more the one
   grows, the more the other decreases, and the remembrance of it
   likewise. If that sensual love grows, it will at once be observed that
   the soul's love of God is becoming colder, and that it is forgetting
   Him as it remembers that love; there comes to it, too, a certain
   remorse of conscience. And, on the other hand, if the love of God grows
   in the soul, that other love becomes cold and is forgotten; for, as the
   two are contrary to one another, not only does the one not aid the
   other, but the one which predominates quenches and confounds the other,
   and becomes strengthened in itself, as the philosophers say. Wherefore
   Our Saviour said in the Gospel: That which is born of the flesh is
   flesh, and that which is born of the Spirit is spirit.' [38] That is to
   say, the love which is born of sensuality ends in sensuality, and that
   which is of the spirit ends in the spirit of God and causes it to grow.
   This is the difference that exists between these two kinds of love,
   whereby we may know them.

   8. When the soul enters the dark night, it brings these kinds of love
   under control. It strengthens and purifies the one, namely that which
   is according to God; and the other it removes and brings to an end; and
   in the beginning it causes both to be lost sight of, as we shall say
   hereafter.