When fasting, every hunger pang reminds us of God—the one for whom we have made this sacrifice. By constantly remembering Allah and sacrificing for Him, we are made more aware of His presence, and in that way we increase our taqwa (fear and consciousness of Him). The same thing that prevents us from the sin of sneaking in food while no one else is watching trains us to avoid other sins while no one else is watching. That is taqwa.
