The Catechism of the Summa - XXXVII. OF FRIENDSHIP; AND OF THE OPPOSITE SINS, NAMELY, OF DISPUTE AND FLATTERY

Is there any other debt that binds only morally, the acquittance of which helps in a great degree towards the welfare of society, although not with the same rigour as that of gratitude, retributive justice, and truthfulness?

Yes; and this is the debt of friendship (CXIV. 2).
 
What is friendship?

It is that virtue which makes man endeavour by the whole of his exterior, both in word and deed, to treat his fellow-beings as it behoves in order to bring mutual pleasantness and charm to their lives (CXIV. 1).
 
Is this a virtue of great price?

Yes, this is a social virtue of great worth; and it might fittingly be called the flower of the virtues of justice and of charity.
 
In what way is it possible to sin against this virtue?

In two ways: by defect, in troubling ourselves little or not at all with what may bring pleasure or annoyance to others; or by excess, and this is the sin of flattery, which fails in disapproving externally the words or deeds of those with whom we live that deserve reproval (CXV.-CXVI.).

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