佛說盂蘭盆經疏 2.86

2.86

雖貴賤品隔、僧俗道殊，自非化生濕生，無不有父有母。慈烏鸚鵡尚解思恩，豈況人倫而不濟拔？孝之利害已具玄談，既識是非須依正道，故云應先為所生等也。據制令必為，不為即是違制，故亦當於制教，是以前判亦屬律藏。然佛無悲喜，今於此日示現歡喜者，應機緣也。以佛本出世只為勸人修行，見人造惡則悲、見人修善則喜，今比丘九旬加行日滿倍更懇誠。三千界中皆同如此稱佛本意，寧不欣歡。此日設供，其福甚矣。

Although the noble and base are separated in rank and the monastic and lay paths are distinct, unless one is born through transformation or moisture, none is without father and mother. Even the filial crow and the parrot still understand reflecting upon kindness; how much more should human beings not rescue and deliver [their parents]? The benefits and detriments of filial piety have already been fully treated in the profound discourse; having recognized right and wrong, one must rely on the correct path. Therefore it says, "they should all first, for the sake of the parents who gave birth to them", and so forth. According to the injunction, one is mandated to do it; not doing it is violating the injunction. Therefore it also pertains to the disciplinary teachings; hence it was previously judged as also belonging to the Vinaya Piṭaka. However, the Buddha has neither grief nor joy; that he manifests joy on this day is in response to capacities and conditions. Because the Buddha's original appearance in the world was only to encourage people to practice: seeing people create evil he grieves, and seeing people cultivate good he rejoices. Now, the monks' ninety-day intensive practice has completed its days with multiplied sincerity. Throughout the three thousand realms all are alike in this, matching the Buddha's original intent; how could he not be delighted? Setting out offerings on this day, its merit is immense!