瑜伽論記 17.393

17.393

言相對者，牒經也。謂有此故，施設於彼，名相對等者，待苦說樂待憂說喜施設五受，然由行苦一切皆苦，故云此中諸受展轉相對。

As for 'Relative', it brings up the sūtra. As for 'Because this exists, that is designated, which is called relative, etc.', the five feelings are designated by depending on pain to speak of pleasure, and depending on grief to speak of joy; yet, due to the suffering of conditioned formations, all are suffering, which is why it says 'Herein, feelings are mutually relative'.