When Ravan was destroying the Dharma of all the three worlds, all the celestials along with Brahma Ji went and sought refuge with Bhagwan Vishnu. Listening to all their pleas, Bhagwan Vishnu says he would be born in the Ikshvaku dynasty as the son of Dasarath and Kaushalya, and then through his divine Lila, kills Ravan. The sloka in Shri Ramcharitramanas mentions as to why he chose to be born as their son.
कस्यप अदिति महािप कीन्हा। तिन्ह कह ुँ मैं पर
ब बर िीन्हा॥
िे िसरथ कौसल्या रूपा। कोसलपर ीीं प्रगट नर भपू ा॥2
To fulfil the boon given to Kashyap and Aditi who were reborn as Dasarath and Kaushalya, Bhagwan Vishnu chose to be born as their son. Few sources also mention Dasarath to be a Brahmin in the Satya Yug who had done great penance about Bhagwan Vishnu. When Bhagwan Vishnu appeared before him, the Brahmin wished for the pure affection of the Almighty and nothing else. Pleased with this, Bhagwan Vishnu says that the Brahmin would he would be born as Dasarath and he as Shri Ram in the Treta Yug, and their bond of father and son would be remembered as one of the greatest bond of pure affection till eternity.
Thus, Dasarath was the son of Aja Maharaj and Indumati. He was named Nemi by his parents, but got the title of Dasarath for three reasons. The first, as his fame was spread in all the ten directions, the second because he was excellent chariot rider, and third, as his son would kill Dashaanan, another name for Ravan.
There is also another reason as to why Bhagwan Vishnu chose to be born in the Ikshavaku dynasty. Ravan once comes across the Ikshvaku king Anaranya, who was at that time, performing a sacred Yagna. Ravan with his arrogance disturbs the yagna. The king becomes furious at this act, curses Ravan that he would be killed by an Ikshvaku warrior in the future.
Ravan later comes to know that the son of Dasarath would kill him. He also comes to know that Dasarath was getting married to Kaushalya, the princess of Magadh. Ravan abducts her before marriage and locks her in a wooden box and throws it in the sea. Meanwhile, Dasarath was on a sea voyage and was hurriedly returning back to his kingdom as he was getting late for his wedding ceremonies. He notices the wooden box floating in the waters and pulls it onto the ship. When he opens the box, he is surprised to find his bride in the box. At that moment, all the celestials appear in front of them and let them know that Bhagwan Vishnu will be born as their son and they were the destined couple chosen by the Almighty, and perform their marriage then and there itself.
Later through the Putrakamesthi yagna, they are blessed with Shri Ram as their son. Dasarath’s other wives, Kaikeyi and Sumitra are also blessed with sons from the insignia of Bhagwan Vishnu. Bharat is born from the Sudarshan Chakr, (the discus), Lakshman (Seshnag) and Shatrughan from the Panchajany (the divine conch).
The rest is history as to how the four brothers have stood together, destroyed the evil forces and established the true ideals of Dharma, bringing glory to their whole dynasty. Indeed blessed were king Dasarath and his queens who have borne none other than the Almighty as their son.