Objection 1. It would seem that the sorrow of sympathizing friends does not assuage our own sorrow. For contraries have contrary effects. Now as Augustine says (Confess. viii, 4), "when many rejoice together, each one has more exuberant joy, for they are kindled and inflamed one by the other." Therefore, in like manner, when many are sorrowful, it seems that their sorrow is greater.
Objection 2. Further, friendship demands mutual love, as Augustine declares (Confess. iv, 9). But a sympathizing friend is pained at the sorrow of his friend with whom he sympathizes. Consequently the pain of a sympathizing friend becomes, to the friend in sorrow, a further cause of sorrow: so that, his pain being doubled his sorrow seems to increase.
Objection 3. Further, sorrow arises from every evil affecting a friend, as though it affected oneself: since "a friend is one's other self" (Ethic. ix, 4,9). But sorrow is an evil. Therefore the sorrow of the sympathizing friend increases the sorrow of the friend with whom he sympathizes.
On the contrary, The Philosopher says (Ethic. ix, 11) that those who are in pain are consoled when their friends sympathize with them.
I answer that, When one is in pain, it is natural that the sympathy of a friend should afford consolation: whereof the Philosopher indicates a twofold reason (Ethic. ix, 11). The first is because, since sorrow has a depressing effect, it is like a weight whereof we strive to unburden ourselves: so that when a man sees others saddened by his own sorrow, it seems as though others were bearing the burden with him, striving, as it were, to lessen its weight; wherefore the load of sorrow becomes lighter for him: something like what occurs in the carrying of bodily burdens. The second and better reason is because when a man's friends condole with him, he sees that he is loved by them, and this affords him pleasure, as stated above (Question 32, Article 5). Consequently, since every pleasure assuages sorrow, as stated above (Article 1), it follows that sorrow is mitigated by a sympathizing friend.
Reply to Objection 1. In either case there is a proof of friendship, viz. when a man rejoices with the joyful, and when he sorrows with the sorrowful. Consequently each becomes an object of pleasure by reason of its cause.
Reply to Objection 2. The friend's sorrow itself would be a cause of sorrow: but consideration of its cause, viz. his love, gives rise rather to pleasure.
And this suffices for the reply to the Third Objection.
Source: Summa Theologica (New Advent)