Objection 1. It would seem that the reason for taking pity is not a defect in the person who takes pity. For it is proper to God to be merciful, wherefore it is written (Psalm 144:9): "His tender mercies are over all His works." But there is no defect in God. Therefore a defect cannot be the reason for taking pity.
Objection 2. Further, if a defect is the reason for taking pity, those in whom there is most defect, must needs take most pity. But this is false: for the Philosopher says (Rhet. ii, 8) that "those who are in a desperate state are pitiless." Therefore it seems that the reason for taking pity is not a defect in the person who pities.
Objection 3. Further, to be treated with contempt is to be defective. But the Philosopher says (Rhet. ii, 8) that "those who are disposed to contumely are pitiless." Therefore the reason for taking pity, is not a defect in the person who pities.
On the contrary, Pity is a kind of sorrow. But a defect is the reason of sorrow, wherefore those who are in bad health give way to sorrow more easily, as we shall say further on (35, 1, ad 2). Therefore the reason why one takes pity is a defect in oneself.
I answer that, Since pity is grief for another's distress, as stated above (Article 1), from the very fact that a person takes pity on anyone, it follows that another's distress grieves him. And since sorrow or grief is about one's own ills, one grieves or sorrows for another's distress, in so far as one looks upon another's distress as one's own.
Now this happens in two ways: first, through union of the affections, which is the effect of love. For, since he who loves another looks upon his friend as another self, he counts his friend's hurt as his own, so that he grieves for his friend's hurt as though he were hurt himself. Hence the Philosopher (Ethic. ix, 4) reckons "grieving with one's friend" as being one of the signs of friendship, and the Apostle says (Romans 12:15): "Rejoice with them that rejoice, weep with them that weep."
Secondly, it happens through real union, for instance when another's evil comes near to us, so as to pass to us from him. Hence the Philosopher says (Rhet. ii, 8) that men pity such as are akin to them, and the like, because it makes them realize that the same may happen to themselves. This also explains why the old and the wise who consider that they may fall upon evil times, as also feeble and timorous persons, are more inclined to pity: whereas those who deem themselves happy, and so far powerful as to think themselves in no danger of suffering any hurt, are not so inclined to pity.
Accordingly a defect is always the reason for taking pity, either because one looks upon another's defect as one's own, through being united to him by love, or on account of the possibility of suffering in the same way.
Reply to Objection 1. God takes pity on us through love alone, in as much as He loves us as belonging to Him.
Reply to Objection 2. Those who are already in infinite distress, do not fear to suffer more, wherefore they are without pity. On like manner this applies to those also who are in great fear, for they are so intent on their own passion, that they pay no attention to the suffering of others.
Reply to Objection 3. Those who are disposed to contumely, whether through having been contemned, or because they wish to contemn others, are incited to anger and daring, which are manly passions and arouse the human spirit to attempt difficult things. Hence they make a man think that he is going to suffer something in the future, so that while they are disposed in that way they are pitiless, according to Proverbs 27:4: "Anger hath no mercy, nor fury when it breaketh forth." For the same reason the proud are without pity, because they despise others, and think them wicked, so that they account them as suffering deservedly whatever they suffer. Hence Gregory says (Hom. in Evang. xxxiv) that "false godliness," i.e. of the proud, "is not compassionate but disdainful."
Source: Summa Theologica (New Advent)