大般涅槃經義記 2.14

2.14

初中「快說無常苦」等，反嘆昔教。比丘何因歎佛昔說？由佛今滅。佛今定滅，明法定是無常苦空。法既如是，昔言當法，是以言快。何故嘆者？為顯自己所得無常苦空法真，使佛翻之，更顯真法。

In the first section, "Delightfully expounded impermanence and suffering" and so forth inversely praises the past teaching. Why do the bhikṣus praise the Buddha's past exposition? Because the Buddha is now to pass into extinction. Since the Buddha is now definitely to pass into extinction, this clarifies that dharmas are definitely impermanent, suffering, and empty. Since dharmas are indeed like this, the past words corresponded to the reality of dharmas; therefore, they say "delightfully [expounded]". Why do they praise it? In order to show that the dharma of impermanence, suffering, and emptiness which they themselves attained is real, so as to cause the Buddha to overturn it and further reveal the true Dharma.