from Ascent to Love
The whole aim of John's asceticism is to release us from the tyranny of the ego. Influenced by his scholastic framework he seems to write as if the senses had a life of their own and must control their actions; that the appetites, passions and emotions must likewise curb themselves. But of course, this is not so. It is really the will, the faculty of choosing, that is involved. True, the eye sees, the ear hears automatically; passions are aroused automatically, but it is the will that must choose to turn away the eyes, refuse to listen, control the instincts. Everything therefore will depend on what I really want, what I prize, what I hold to be my true good. Meditation, as we have said, keeps us looking at the values of Jesus so that we may choose to make these our own. Jesus is always summoning us beyond ourselves to the Father, bidding us deny the powerful tendency to seek fulfilment within ourselves and the limits of the created, making the aggrandisement of the ego the implicit motivation of our thinking and acting.
Throughout life at different times each of us faces the trials experienced by Jesus in Luke 4.
And Jesus, full of the Holy Spirit, returned from the Jordan, and was led by the Spirit  for forty days in the wilderness, tempted by the devil. And he ate nothing in those days; and when they were ended, he was hungry.  The devil said to him, "If you are the Son of God, command this stone to become bread."  And Jesus answered him, "It is written, `Man shall not live by bread alone.'"  And the devil took him up, and showed him all the kingdoms of the world in a moment of time,  and said to him, "To you I will give all this authority and their glory; for it has been delivered to me, and I give it to whom I will.  If you, then, will worship me, it shall all be yours."  And Jesus answered him, "It is written, `You shall worship the Lord your God, and him only shall you serve.'"
 And he took him to Jerusalem, and set him on the pinnacle of the temple, and said to him, "If you are the Son of God, throw yourself down from here;  for it is written, `He will give his angels charge of you, to guard you,'  and `On their hands they will bear you up, lest you strike your foot against a stone.'"  And Jesus answered him, "It is said, `You shall not tempt the Lord your God.'"  And when the devil had ended every temptation, he departed from him until an opportune time. And Jesus returned in the power of the Spirit into Galilee, and a report concerning him went out through all the surrounding country.
Unfortunately, more often than not, we do not respond as Jesus does. For a variety of reasons, different reasons at different times, we succumb to the temptations offered. The reason for meditating on the Scriptures and for practicing a certain level of self denial is to prepare us for the time when these temptations present themselves. Jesus "practiced" self-control and self-denial in a marathon 40 day fast in the desert. He withdrew from all of the wonderful things of God's creation--food, wine, people, comfortable lodging, everything that we see as the necessary minimum in life. This prepared Him for answering Satan when the temptation was offered.
Now few of us are up to a complete fast for even a single day. The thought of a pang of hunger is enough to send us running to our pantries to check out our famine supplies. But neither God nor St. John of the Cross is telling us that it is a really good idea to fast for forty days. In fact, for some of us that presents a temptation all its own--the temptation to being "holier than thou." A kind of spiritual "extreme sports." 'I can fast longer than you can AND I can sit on a taller pole in a higher wind.' "Well I'll take your fast and raise you a 10 cord discipline twice a day.' It sounds silly, but people being what they are seem to be able to take pride in just about anything.
What we learn from St. John of the Cross is that we do well to deprive ourselves of small luxuries, things that in the normal course of life no one will notice except God. Then we are neither likely to take pride in them--so long as we do not deliberately bring them to the notice of others--nor are they likely to derail us by their sheer heroism. In fact, the are more likely to reinforce humility when we realize the tremendous effort we must take to momentarily deprive ourselves of something we don't really need anyway.
And all of this is about conforming the will to what God would have us do. We must make the choices, we must take action--but our action must conform to God's plan for us for it to mean anything. And this is the purpose of any self-denial or any discipline we impose. If our goal is anything less than total-self-giving to God, our actions will not have their intended consequences. As Sister Ruth points out, we must make the choice for our own greatest good. And the difficulty there is that we must wake up and come to realize what our own greatest good entails. Meditating on the scriptures will help us to open our eyes and to see what is right there in front of us, rather than what is six years (six years we don't have) down the line either direction.