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Gibbon seems to be commenting on human nature and motivation. The quote suggests that seeking revenge is personally or politically advantageous, as it can satisfy a desire for retaliation or demonstrate strength.
However, expressing gratitude requires sacrificing something of value, whether that be pride, money, or political capital. So while revenge yields profit, gratitude tends to cost something extra.
Overall Gibbon appears to be making a cynical observation about how self-interest can undermine virtues like forgiveness and appreciation.