The Eternal Father speaks to Catherine:

I have told you how tears well up from the heart: The heart gathers them up from its burning desire and holds them out to the eyes. Just as green wood, when it is put into the fire, weeps tears of water in the heat because it is still green (for if it were dry it would not weep), so does the heart weep when it is made green again by the renewal of grace, after the desiccating dryness of selfishness has been drawn out of the soul. Thus are fire and tears made one in burning desire. And because desire has no end it cannot be satisfied in this life. Rather, the more it loves, the less it seems to itself to love. So love exerts a holy longing, and with that longing the eyes weep.

But once the soul is separated from the body and has reached Me, her final goal, she does not on that account give up her desire so as to no longer desire Me and the charity of her neighbours. For charity has entered into her like a great lady, bearing with her the fruit of all the other virtues. What has ended is suffering, because if she longs for Me she now possesses me in truth without any fear of being able to lose what she has so long desired. This is how she feeds the flame, for the more she hungers the more she is filled, and the more she is sated, the more she hungers. … So your desire is an infinite thing. Were it not, could I be served by any finite thing, no virtue would have value or life. For I, who am infinite God, want you to serve Me with what is infinite, and you have nothing infinite except your soul’s love and desire.

From the Dialogue of St Catherine of Siena – Chapter 92