For a while I had a habit of fasting on Sundays: I'd skip lunch. But why did I do it? Well, I didn't have a valid reason, and I stopped after a while.
So what is a valid reason? Merton helps us again here:
… The perfection of Christian renunciation is the total offering of ourselves to God in union with the sacrifice of Christ. …So that's the point: to gain control over our hearts and their passions, so that we can become indifferent to food or whatever we're attracted (or addicted) to, so that we can get closer to the ideal: to offer our whole selves to God, to be indifferent to everything except the will of God.
To offer this sacrifice perfectly we must practice asceticism, without which we cannot gain enough control over our hearts and their passions to reach such a degree of indifference to life and death.Merton, op. cit., p. 102
And that's a valid reason: to be free of our own appetites and passions, not just regarding food, but anything that one might be (or become) addicted to. As Paul writes to married couples in 1 Corinthians 7.5, which I had not understood before: Do not deprive each other except by mutual consent and for a time, so that you may devote yourselves to prayer.
The point, of course, isn't that husbands and wives spend so much time in bed that they have no time to pray; rather, it's that by abstaining voluntarily for a (brief) time they can surrender more of themselves to God.
For a brief time, that is.