not to reciprocate is against etiquette; it is polite to return a favor or a visit

所谓“来而不往非礼也”，你送了我礼物，我也得回送一份。

to treat someone in the same way they have treated you, often used in the context of retaliation

既然对方先发起了挑战，来而不往非礼也，我们也该予以回击。