There are three principal reasons why we find ourselves desolate.Is it news that a woman so far along the path to sainthood was being taught advanced lessons in the spiritual life? I suppose ignorance about basic Christianity ensures it is.
The first is, because of our being tepid, lazy or negligent in our spiritual exercises; and so through our faults, spiritual consolation withdraws from us.
The second, to try us and see how much we are and how much we let ourselves out in His service and praise without such great pay of consolation and great graces.
The third, to give us true acquaintance and knowledge, that we may interiorly feel that it is not ours to get or keep great devotion, intense love, tears, or any other spiritual consolation, but that all is the gift and grace of God our Lord, and that we may not build a nest in a thing not ours, raising our intellect into some pride or vainglory, attributing to us devotion or the other things of the spiritual consolation.
update: For more extended treatment, do consult this article from 2003:
We may prefer to think that she spent her days in a state of ecstatic mystical union with God, because that would get us ordinary worldlings off the hook. How else could this unremarkable woman, no different from the rest of us, bear to throw her lot in with the poorest of the poor, sharing their meager diet and rough clothing, wiping leprous sores and enduring the agonies of the dying, for so many years without respite, unless she were somehow lifted above it all, shielded by spiritual endorphins? Yet we have her own testimony that what made her self-negating work possible was not a subjective experience of ecstasy but an objective relationship to God shorn of the sensible awareness of God's presence...
Mother Teresa learned to deal with her trial of faith by converting her feeling of abandonment by God into an act of abandonment to God. It would be her Gethsemane, she came to believe, and her participation in the thirst Jesus suffered on the Cross. And it gave her access to the deepest poverty of the modern world: the poverty of meaninglessness and loneliness. To endure this trial of faith would be to bear witness to the fidelity for which the world is starving. "Keep smiling," Mother Teresa used to tell her community and guests, and somehow, coming from her, it doesn't seem trite. For when she kept smiling during her night of faith, it was not a cover-up but a manifestation of her loving resolve to be "an apostle of joy."