來而不往非禮也

lái'érbùwǎngfēilǐyě

it is impolite not to reciprocate—one should return as good as one receives

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來而不往非禮也|来而不往非礼也 [lái ér bù wǎng fēi lǐ yě] ► not to reciprocate is against etiquette (classical) ► to respond in kind