The story of Valmiki writing the Ramayana, after seeing a bird shot down by an arrow is a familiar one. But why did Valmiki's sorrow result in the Ramayana? Valmiki was specially chosen by the Lord to write His story, said Adur Asuri Madhavachari in a discourse.
In the Skanda Purana, a conversation between Lord Brahma and Lord Rama is mentioned. The purpose of the Rama avatara has been accomplished with the killing of Ravana. So Brahma comes to Rama to remind the latter that it is time to end the avatara. But Rama says He wants to have a private conversation with Brahma, and sends away all His servants and attendants. Usually, if it were matters of dharma, Vasishtha would be consulted. If it were matters of state, Sumantra would be consulted. But now Rama does not even have them with Him. He tells Brahma that His story should be written, so that it will benefit those who are caught up in samsara, and suffer as a result of it. People may want to get out of samsara, but do not know how they can attain this. It is for them that His story should be told, says Rama. Those who read it, or listen to it, will learn the means to liberation. The story will be like a boat that helps them cross the ocean of samsara. But a suitable person had to be found to write it. Rama tells Brahma that that person should be Valmiki.
Valmiki is a man with a tender heart, and one who empathises with the sufferings of others. Moreover, he has sought spiritual guidance from sage Narada. So He should write Rama's story, it is decided, by none other than Rama himself, and He conveys this to Brahma.
Thondaradippodi Azhvar says that if a person's lifespan were a hundred years he would spend half of it in sleep. And then he has to make provisions for meeting his needs, and then he suffers from diseases and old age. Where then is the time for him to think of God? Relieve me, therefore, from repeated births, Thondaradippodi Azhvar prays to Lord Ranganatha of Srirangam. Thirumangai Azhvar says that samsara is like a poisonous tree. How we can be rid of samsara is shown in the Ramayana. And Valmiki was chosen by God Himself to write it.