Objection 1. It would seem that the blessed pity the unhappiness of the damned. For pity proceeds from charity [Cf. II-II, 30]; and charity will be most perfect in the blessed. Therefore they will most especially pity the sufferings of the damned.
Objection 2. Further, the blessed will never be so far from taking pity as God is. Yet in a sense God compassionates our afflictions, wherefore He is said to be merciful.
On the contrary, Whoever pities another shares somewhat in his unhappiness. But the blessed cannot share in any unhappiness. Therefore they do not pity the afflictions of the damned.
I answer that, Mercy or compassion may be in a person in two ways: first by way of passion, secondly by way of choice. In the blessed there will be no passion in the lower powers except as a result of the reason's choice. Hence compassion or mercy will not be in them, except by the choice of reason. Now mercy or compassion comes of the reason's choice when a person wishes another's evil to be dispelled: wherefore in those things which, in accordance with reason, we do not wish to be dispelled, we have no such compassion. But so long as sinners are in this world they are in such a state that without prejudice to the Divine justice they can be taken away from a state of unhappiness and sin to a state of happiness. Consequently it is possible to have compassion on them both by the choice of the will--in which sense God, the angels and the blessed are said to pity them by desiring their salvation--and by passion, in which way they are pitied by the good men who are in the state of wayfarers. But in the future state it will be impossible for them to be taken away from their unhappiness: and consequently it will not be possible to pity their sufferings according to right reason. Therefore the blessed in glory will have no pity on the damned.
Reply to Objection 1. Charity is the principle of pity when it is possible for us out of charity to wish the cessation of a person's unhappiness. But the saints cannot desire this for the damned, since it would be contrary to Divine justice. Consequently the argument does not prove.
Reply to Objection 2. God is said to be merciful, in so far as He succors those whom it is befitting to be released from their afflictions in accordance with the order of wisdom and justice: not as though He pitied the damned except perhaps in punishing them less than they deserve.
Source: Summa Theologica (New Advent)