Objection 1. It seems that quarreling is a less grievous sin than the contrary vice, viz. adulation or flattery. For the more harm a sin does the more grievous it seems to be. Now flattery does more harm than quarreling, for it is written (Isaiah 3:12): "O My people, they that call thee blessed, the same deceive thee, and destroy the way of thy steps." Therefore flattery is a more grievous sin than quarreling.
Objection 2. Further, there appears to be a certain amount of deceit in flattery, since the flatterer says one thing, and thinks another: whereas the quarrelsome man is without deceit, for he contradicts openly. Now he that sins deceitfully is a viler man, according to the Philosopher (Ethic. vii, 6). Therefore flattery is a more grievous sin than quarreling.
Objection 3. Further, shame is fear of what is vile, according to the Philosopher (Ethic. iv, 9). But a man is more ashamed to be a flatterer than a quarreler. Therefore quarreling is a less grievous sin than flattery.
On the contrary, The more a sin is inconsistent with the spiritual state, the more it appears to be grievous. Now quarreling seems to be more inconsistent with the spiritual state: for it is written (1 Timothy 3:2-3) that it "behooveth a bishop to be... not quarrelsome"; and (2 Timothy 3:24): "The servant of the Lord must not wrangle." Therefore quarreling seems to be a more grievous sin than flattery.
I answer that, We can speak of each of these sins in two ways. On one way we may consider the species of either sin, and thus the more a vice is at variance with the opposite virtue the more grievous it is. Now the virtue of friendship has a greater tendency to please than to displease: and so the quarrelsome man, who exceeds in giving displeasure sins more grievously than the adulator or flatterer, who exceeds in giving pleasure. On another way we may consider them as regards certain external motives, and thus flattery sometimes more grievous, for instance when one intends by deception to acquire undue honor or gain: while sometimes quarreling is more grievous; for instance, when one intends either to deny the truth, or to hold up the speaker to contempt.
Reply to Objection 1. Just as the flatterer may do harm by deceiving secretly, so the quarreler may do harm sometimes by assailing openly. Now, other things being equal, it is more grievous to harm a person openly, by violence as it were, than secretly. Wherefore robbery is a more grievous sin than theft, as stated above (Question 66, Article 09).
Reply to Objection 2. In human acts, the more grievous is not always the more vile. For the comeliness of a man has its source in his reason: wherefore the sins of the flesh, whereby the flesh enslaves the reason, are viler, although spiritual sins are more grievous, since they proceed from greater contempt. On like manner, sins that are committed through deceit are viler, in so far as they seem to arise from a certain weakness, and from a certain falseness of the reason, although sins that are committed openly proceed sometimes from a greater contempt. Hence flattery, through being accompanied by deceit, seems to be a viler sin; while quarreling, through proceeding from greater contempt, is apparently more grievous.
Reply to Objection 3. As stated in the objection, shame regards the vileness of a sin; wherefore a man is not always more ashamed of a more grievous sin, but of a viler sin. Hence it is that a man is more ashamed of flattery than of quarreling, although quarreling is more grievous.
Source: Summa Theologica (New Advent)