Thomas Aquinas, Compendium of Theology

It also happens that some men depart this life free from mortal sin but nevertheless stained with venial sin. The commission of such sins does not, indeed, turn them from their last end; but by committing them they have erred with regard to the means leading to the end, out of undue attachment to those means. In the case of some perfect men sins of this kind are expiated by the fervor of their love. But in others these sins must be atoned for by punishment of some sort; no one is admitted to the possession of eternal life unless he is free from all sin and imperfection. Therefore we must acknowledge the existence of purgatorial punishment after this life.

Such punishments derive their cleansing power from the condition of those who suffer them. For the souls in purgatory are adorned with charity, by which their wills are conformed to the divine will; it is owing to this charity that the punishments they suffer avail them for cleansing.

This is why punishment has no cleansing force in those who lack charity, such as the damned. The defilement of their sin remains forever, and so their punishment endures forever.