Now self-love includes inordinate desire of good: for a man desires good for the one he loves. Hence it is evident that inordinate desire of good is the cause of every sin.
 Thomas Aquinas, Summa Theologica (1274). copy citation

add
Author Thomas Aquinas
Source Summa Theologica
Topic sin love
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

“Now a thing is said to be "in the world" by reason of sin: wherefore it is written (1 John 5:19) : "The whole world is seated in wickedness." Therefore these three are causes of sin.
I answer that, As stated above (A. 4) , inordinate self-love is the cause of every sin. Now self-love includes inordinate desire of good: for a man desires good for the one he loves. Hence it is evident that inordinate desire of good is the cause of every sin. Now good is, in two ways, the object of the sensitive appetite, wherein are the passions which are the cause of sin: first, absolutely, according as it is the object of the concupiscible part; secondly, under the aspect of difficulty, according as it is the object of the irascible part, as stated above (Q. 23, A. 1).” source