Why did Song Poetry emphasize "rational interest"?

The so-called rational interest refers to the philosophy of life embodied in poetic images. The cultural root of the formation of rational interest in Song poetry is mainly Buddhism rather than Neo-Confucianism. Wang Anshi, Su Shi and their followers, Huang Tingjian and Chen Shidao, made the most rational and interesting representative works in the Song Dynasty, and their philosophical thoughts had formed their own system before the formation of Neo-Confucianism. For example, Su Shi's "A Poem with Rhyme and Method" said: "Where spring comes, it does not return to the flood, nor does it return."