Mortification and fasting– unlike suffering and starvation– are deliberate, conscious activities. We reenact Christ’s passion when we undertake the deaths represented by mortification and fasting. We join with Christ in proclaiming the end of death’s dominion.

The evil of suffering, in the mystery of Christ’s redemption, is overcome and in every case transformed. It becomes a force of liberation from evil, for the victory of the good. All human sufferings, united to that of Christ, complete “what is lacking in Christ’s afflictions for the sake of his body” (cf. Col 1:24). The body is the Church as the universal community of salvation.
John Paul II

For the Christian the only appropriate way to cope with the negative is in the light of the cross. Following the cross does not mean cultic adoration, mystical absorption, or ethical imitation. It means practice in a variety of ways in accordance with the cross of Jesus, in which a person freely perceives and attempts to follow his own way of life and suffering.
Hans Kung