Friendship is the source of ...

Friendship is the source of the greatest pleasures, and without friends even the most agreeable pursuits become tedious.
 Thomas Aquinas

Quotes from the same author

There is nothing on this earth more to be prized than true friendship.
 Thomas Aquinas
A person is disposed to an act of choice by an angel ... in two ways. Sometimes, a man's understanding is enlightened by an angel to know what is good, but it is not instructed as to the reason why ... But sometimes he is instructed by angelic illumination, both that this act is good and as to the reason why it is good.
 Thomas Aquinas
A man does not always choose what his guardian angel intends.
 Thomas Aquinas
Given the sin of impiety through which they [the Romans] sinned against the divine nature [by idolatry], the punishment that led them to sin against their own nature followed.... I say, therefore, that since they changed into lies [by idolatry] the truth about God, He brought them to ignominious passions, that is, to sins against nature; not that God led them to evil, but only that he abandoned them to evil.
 Thomas Aquinas
Charity is the form, mover, mother and root of all the virtues.
 Thomas Aquinas