Suffering is relative. A person who suffers without hope of relief is indeed a sad case. But one who knows that suffering will end in perfect freedom from pain or disease gladly endures the severest pain. A patient of an incurable disease finds no solace in suffering, nothing but the dead weight of pain. But a patient who has the assurance of recovery willingly endures the surgeon's knife or the unpleasant remedies of the physician.

Purgatory is the vestibule of Heaven. It is the certainty of eternal salvation. All its sufferings are inflicted in love and endured in love. It is because the souls in Purgatory love God so much that they suffer so much. In purgatory they realize what God is, are drawn to Him most powerfully, and yet repelled from Him by the state of their souls. They cannot themselves hasten their union with God, since the time of meriting ends with life. But they can be helped by the charity of their friends on earth.

By the communion of saints we on earth can offer to God our prayers, good works, alms-deeds and sacrifices in their behalf. Above all, we can assist at the holy sacrifice of the Mass for them, and best of all, we can have the holy sacrifice offered especially for them. When St. Monica the mother of St. Augustine felt that she was about to die she said to her son:
"Lay this body of mine where thou wilt; one thing only I ask of thee, remember me when I am gone, at the holy sacrifice of the altar." This spoke one saint to another, in the first ages of Christianity!

By praying for the souls in Purgatory the Christian practices his religion in a most salutary manner both for himself and for those for whom he prays.

In aiding the faithful departed we are practicing first of all an act of charity. We are doing something for those who are beyond self-help.This act of charity is also an act of love of God, for it is done for love of God and to unite to Him the sooner those who are dear to Him. Of course God could, if He wished, terminate the period of suffering in Purgatory at any time, but we must remember that God is just as well as merciful. He shows His justice by requiring chastisement for wrong-doing, and His mercy by permitting us to go to the aid of those being chastised.

By our faith and charity we can offer to God satisfaction for the souls in Purgatory, and trust to the goodness of God to accept our satisfaction for them in the most generous measure compatible with justice. For, after all, when we pray for the faithful departed, it rests with God to accept our prayers and good works in their behalf.

A lifelong sinner, but one who repented at the last, might have a thousand Masses offered for his soul by the generosity of his friends, or by a stipulation in his will, yet Almighty God might see fit not to accept them in his behalf. Such a soul, forgiven by God's mercy, might have to suffer chastisement for his sins to the day of judgment. One cannot purchase one's way to heaven. In praying for the faithful departed we do so by way of suffrage, that is, we offer our good works to God in their behalf, leaving it to Him to accept them for those for whom they are offered.

Of this however, we can be certain, that since He invites us to pray for them, no prayer of ours in their behalf goes without giving them some assistance. The doctrine of Purgatory is a great help to the living, since it enables them to practice charity in a high degree, and a consolation to the souls in Purgatory since it gives them hope that their period of probation may be shortened. It also helps the practice of faith, by causing us to make sacrifices in our daily life in accordance with the teaching of faith; and finally it aids in the practice of Christian hope, because we trust that the good God will not only be merciful to those for whom we pray, but to ourselves also, who for love of Him pray for those who love Him and whom He loves and whom He desires to welcome home as soon as they are made ready for His loving embrace. "It is a holy and wholesome thought to pray for the dead that they may be loosed from their sins."

Source: "Things Catholics are asked about" by Fr. Martin J. Scott, S.J.Litt.D.