The dissention that rises against spiritual disciplines is that they ENHANCE the already-deadly hyper-individualization of discipleship. I agree, as do many others, that this is a struggle and I feel the reality of it sometimes in conversations about belief and practice within the church. Yet I really side with Robert Mulholland in the fact that "spiritual [discipline] is about being transformed into the image of Christ for the sake of others." This takes the emphasis off of the simple act of transformation, which can become an end unto itself, and replaces the focus onto what impact formation and discipline has on others. If our spiritual formation and discipline have no effect on how we treat other people, both familiar and unfamiliar, then it is not spiritual formation but simple self-congratulating piety that we are practicing.
The major reason behind this is that any of the so-called "classical" disciplines are geared toward transforming relational elements of our character and habit. This is somewhat an echo of the Trinity, the community-nature of God, and the fellowship that is shared within those three persons. It is perfect relational action summed up in the very existence of God. Spiritual formation and discipline is a process of growing into that same relationship, albeit imperfectly, here in the "already."
Practically, when we fast we take away something that causes significant ripples in our personality. When we are fasting (or giving, practicing solitude, simplicity) we realize not only our dependence on food and our ability to turn to food during times of stress, but we also see how we treat other people when we are hungry. We subject our personality to naked scrutiny when we remove that element.
This works because it is when we are at our worst that we find out who we really are--how far we are from the example of Christ. We realize how much patience, resolve, concentration, grace, and forgiveness we have when the headache of missing ONE MEAL kicks in. We also begin to sympathize with the instability and volatility of places in the world where daily hunger is a reality--we're ready to write off our family when we miss a meal, imagine what we would do if we missed a few days? Consequently, this is the selling point for those who recruit for militant groups such as Hamas--food and shelter if you join our cause. Who wouldn't do this to give their family one meal a day?
I think this is the perspective on the spiritual disciplines that will ultimately lead them out of the "personal relationship" arena and into the community/Kingdom building activity that is so desperately needed. The willingness to subject ourselves to ourselves at our worst is the lens needed to see where we must be conformed to Christ and how our worst impacts others.