It's always been a struggle for me in prayer. If I only pray when or because I enjoy it, or worse, because like the idea of prayer, or still worse, because I like the idea of myself as a prayerful person, then my prayer is only a self-devotion. This is what it really means to take God's name in vain. That's why the "night of sense" is a real gift of grace; when the Spirit removes from us the natural interest, desire, and gratification we receive in prayer, then we find out if we really love God for God's sake alone.
This isn't to say that a natural interest in prayer and devotion isn't a good thing, or that it shouldn't make us feel good. If they get us praying in the first place, they're good. But we need to notice that when God takes our interest and "feeling like it" away, this is actually a grace.
When this "night of sense" happens, we have two choices. We can panic over the consolation and apparent fulfillment and good feelings we seem to have lost, and run to fill ourselves up with our drug of choice instead. This is what it means to go back to Egypt. Or we can trust in the God we cannot see, and believe that we are being led obscurely through a place where our feeling have become a desert.
I was so struck by his remark about going back to Egypt vs. trusting the Lord in the wilderness, and the real grace of the "night of sense". This is the true desert of the heart, a place we must learn to live in, even to love; though, like Moses, we may not see the Promised Land in our life on earth.