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) .
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic pain sympathy
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17897/pg17897-images.html

Context

“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.” source