What is the reason for the poet's homesickness in the Tang poem Yugan Hotel?

Birds don't know the change of the valley, and go to Yiyang River at dusk. Yugan county, Raozhou in Tang Dynasty, is now Yugan, Jiangxi. "Ancient County" refers to yugan county built before the Tang Dynasty. In the pre-Qin period, Yu Han, his masterpiece, was named after the excess of water and sweat in the territory, and it was a city in the western border of Yue. In the northwest of Anren River (now xinjiang, Jiangxi), the upper reaches of Anren River belong to the State of Chu, so the poem says "Chu Shui Xi". Hanyu County was built in the Han Dynasty, and was renamed yugan county in the Sui Dynasty. In the Tang Dynasty, the county was moved to govern the city, and the old county was gradually abandoned. This poem by Liu Changqing is a masterpiece mourning the past and hurting the present, and it has become a famous work in the Tang Dynasty. This deserted ancient city became famous far and wide. Later, some people called it "Baiyun City", while others built "Baiyun Pavilion", all of which belonged to Liu Shi. In the second year of Tang Suzong Shangyuan (76 1), Liu Changqing passed Yugan on his way home from Nanba, Panzhou, Lingnan. The poet was demoted because he was framed for official conduct, so he deeply felt the political corruption and the dirty officialdom at that time. Now the region he has experienced has just experienced a scuffle between warlords, and the wounds of war are everywhere, which shows the weakness of the country and the hardships of the people, which makes the poet more worried about the national luck of the Tang Dynasty. This lyric poem contains such deep sighs, loneliness and sadness, deep confusion, feelings in the scene, feelings outside the image, endless thoughts and thought-provoking. This is a small mountain city, standing high by the water, just like an isolated city on a fortress, standing in the western part of Yue State in a trance, just like in the pre-Qin period. It is too high, as if it were as high as the white clouds in the sky; So desolate, it seems that no one has been here for 100 million years. The city is empty, and the former government office building has long been hidden in the thick weeds in autumn. Only the female wall in the city is still there, but the soldiers are no longer patrolling, and only crows are heard in the city at night. Standing at the head of the city, the flat sand is endless and confusing; The lonely sunset, facing the poet, a distant visitor, Ran Ran, fell, and the world was extraordinarily silent. In this lonely world, the poet remembered the Book of Songs. Xiaoya. The poem at the turn of October: "The high shore is the valley, and the deep valley is the mausoleum. Those who mourn today will not be punished. " Isn't the vicissitudes of the ancient city "ancient spiritual change"? The poet was deeply moved by the changes of history. However, ignorant birds don't understand all this, but they still fly here to feed and go back and forth. This poem is lyrical and does not stick to historical facts. In order to highlight the theme, the poet made bold fiction and imagination. In the early Tang Dynasty, the poet abandoned the city and moved it forward to the pre-Qin period. The reason for giving up is the transfer of county administration, which the poet implicitly describes as the decline of the ancient city caused by political corruption. Out of this idea, the second couplet is about the desolation of the city, highlighting that the official residence and the female wall still exist, suggesting that the ancient city was not destroyed by the war. Sanlian wrote that the four fields were desolate and the farmland turned into flat sand. Couplets finally come down to the annihilation of people. With the allusions at the turn of October, it is pointed out that the ancient city was abandoned because of political corruption, which led people to leave their homes and flee everywhere. The old saying "The Turn of October" was written by "the doctor stabbed the king", which strongly accused Zhou Youwang of being dissolute and fatuous, making mistakes in the country and the people, "the evil of the people fell from the sky, and the job struggle was left to people", which led to the ancient disaster, and even "all the people had to escape". Combined with the previous triple description, it can be seen that this is the meaning used here. This is a landscape poem as well as a political lyric poem. The scenery it depicts is historical, not natural. The desolate ancient city is not pleasing to the eye, not the object of appreciation, but just an example of the poet's thoughts and emotions, which makes people think, grieve, remember history and reflect on reality. Therefore, this poem is not only fictional and imaginative in dealing with the subject matter, but also highlights the poet's feelings and self-image in the structure of the poem. The poet is full of concern for the country and people, guiding people to board this high-risk and desolate ancient city, an empty and desolate city, instructing people to pay attention to historical sites that can play a warning role, inspiring people to sing emotionally and prompting people to think deeply. Fang commented on this poem: "The implication is that there are people who go to the city and poets, so they are called authors." (Zhao Wei Zhan Yan) pointedly pointed out the artistic characteristics of this poem.