First, because parents love their children as being part of themselves, whereas the father is not part of his son, so that the love of a father for his children, is more like a man's love for himself.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic love parents
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/18755/pg18755-images.html

Context

“Secondly, the degrees of love may be measured from the standpoint of the lover, and in this respect a man loves more that which is more closely connected with him, in which way a man's children are more lovable to him than his father, as the Philosopher states (Ethic. viii) . First, because parents love their children as being part of themselves, whereas the father is not part of his son, so that the love of a father for his children, is more like a man's love for himself. Secondly, because parents know better that so and so is their child than vice versa. Thirdly, because children are nearer to their parents, as being part of them, than their parents are to them to whom they stand in the relation of a principle.” source