Why does Jian 'an poetry have the significance of connecting the past with the future in the history of poetry?

Cao Cao is always sad, Cao Pi is graceful and restrained, and Cao Zhi is both talented and talented "(Yuan Xingpei's History of China Literature). Jian 'an is the title of Emperor Xian of Han Dynasty (196-22). The works represented by Cao Cao and his son reflect the social unrest and the sufferings of people's livelihood, but also show the ideal and ambition of unifying the world, which has distinct characteristics of the times. The lofty political ideal, the short lament of life, strong personality and strong tragic color constitute the era style of "Jian 'an style" (also called "Han and Wei style"). Cao Shi, the ruler of Wei State, and his son both loved and rewarded literature, recruited scribes and gathered many writers around them. They directly inherited the realistic tradition of Han Yuefu folk songs and set off a poetic climax.