These works, which God effects in the soul by Himself alone, which are the last operations of pure and simple love in which we have no merit, so pierce and inflame the soul that the body which envelops her seems to be hiding a fire, or like one in a furnace, who can find no rest but death. It is true that the divine love which overwhelms the soul gives, as I think, a peace greater than can be expressed; yet this peace does not in the least diminish her pains, nay, it is love delayed which occasions them, and they are greater in proportion to the perfection of the love of which God has made her capable.[Catherine of Genoa and Don Cattaneo Marabotto, The Spiritual Doctrine of Saint Catherine of Genoa, TAN Books (1989), p. 318. This passage is from Chapter XII of the Treatise on Purgatory.]
On Catherine's account, Purgatory is love pent up. In a sense, the souls in purgatory suffer because of the blocks that their life of sins created, and which still remain as consequences of sin even though they themselves have become free of sin. Their love of God has become so intense that the suffering of Purgatory is nothing other than the bursting of love through these blocks, like a river breaking through a dam. It is a voluntary suffering, as well; in Catherine's image, the gates of Heaven are always open. The souls that are in Purgatory are undergoing their purification because they truly love God: they will allow no internal obstacles to their union with God, and because they still need to clear them all away, this is why they suffer: it is the suffering of pure love pent up, delayed in its complete expression. And they endure it patiently because the very same love means that they will settle for nothing less.
Something roughly similar is found in Newman's Dream of Gerontius:
When then—if such thy lot—thou seest thy Judge,
The sight of Him will kindle in thy heart
All tender, gracious, reverential thoughts.
Thou wilt be sick with love, and yearn for Him,
And feel as though thou couldst but pity Him,
That one so sweet should e'er have placed Himself
At disadvantage such, as to be used
So vilely by a being so vile as thee.
There is a pleading in His pensive eyes
Will pierce thee to the quick, and trouble thee.
And thou wilt hate and loathe thyself; for, though
Now sinless, thou wilt feel that thou hast sinn'd,
As never thou didst feel; and wilt desire
To slink away, and hide thee from His sight:
And yet wilt have a longing aye to dwell
Within the beauty of His countenance.
And these two pains, so counter and so keen,—
The longing for Him, when thou seest Him not;
The shame of self at thought of seeing Him,—
Will be thy veriest, sharpest purgatory.
I go before my Judge. Ah! ….
…. Praise to His Name!
The eager spirit has darted from my hold,
And, with the intemperate energy of love,
Flies to the dear feet of Emmanuel;
But, ere it reach them, the keen sanctity,
Which with its effluence, like a glory, clothes
And circles round the Crucified, has seized,
And scorch'd, and shrivell'd it; and now it lies
Passive and still before the awful Throne.
O happy, suffering soul! for it is safe,
Consumed, yet quicken'd, by the glance of God.
St. Catherine puts a much a greater emphasis on the joy and peace of the souls in Purgatory, though.