Fasting reveals the nafs [ego or self]. When fasting, it whispers to us that we really should eat or drink, that we won't have the energy or concentration to continue, that we might even lose our jobs if we insist on fasting. The nafs comes up with all kinds of reasons and excuses to break our fast, and it becomes much more visible than it usually is. We can often see our impatient and selfish side more clearly when we fast.
— Robert Frager, Heart, Self, and Soul