佛說盂蘭盆經疏 2.83

2.83

初句標讚大善快問者，深契聖心；後二句釋所以。以正欲說，即遇問詞，機感相投潛通密應，故言快問。佛本意者，欲說孝道最大故、拔苦事重故、盂蘭法勝故。世尊覩眾勝緣機熟，可教化故。

The first phrase marks the praise, "Excellent! A timely question!", as it deeply accorded with the sage's mind. The latter two phrases explain the reason. Because just as he was wishing to speak, he met with these words of inquiry—capacity and stimulus corresponding with one another, hiddenly connecting and secretly responding—it is therefore called a timely question. As for the Buddha's original intent, it was because he wished to explain that the way of filial piety is the greatest, because the matter of uprooting suffering is grave, and because the Ullambana dharma is supreme. The World-Honoured One saw that the superior conditions of the assembly and their capacities were mature and capable of being taught and transformed.