Adi Sankara is quoted to have said that, “Only through God’s grace may we obtain the three rarest gifts: Human birth; the longing for liberation; and discipleship to an illumined teacher.”
There were three factors that prompted Sankara to leave his home and become a wandering ascetic, writes Madhava-vidyaranya in his book "Sankara-Dig-Vijaya". First was the revelation by Agathiyar reminding Sankara of his mission; second Sankara gaining permission from Sri Krishna and his mother; and above all his intense yearning for renunciation.
During the Treta Yuga, when Sri Rama was wandering as a hermit, many Rishis yearned for Sri Rama to grace their ashrams. As these Rishis were engaged in severe austerities or tapas, they could not leave their dwellings to meet Sri Rama. Sri Rama, on his part, could not visit every single ashram of these Rishis. Sri Rama could only meet Agathiyar, Bharatvajar, and Sabari. Sri Rama too had to move on to Kishkinta, Sundara, and Yutta Kaandams. Hence, he could not grace the ashrams of hundreds of Rishis. These wishes of the Rishis became Vasanas and got attached to Sri Rama. Hence, Sri Rama had to exhaust these Vasanas by granting their wishes in his next incarnation as Sri Krishna. Agathiyar reveals to Sri Rama, "In your next incarnation as Sri Krishna in Dwabara Yuga, these Rishis who had cherished to have your Darshan and serve you, will take rebirth as Gopikas and through play and devotion, and service to you, attain bliss and fulfillment. This is the secret (mystery) behind incarnations and rebirth". (Source: Posts of 17.7.2014 of "Siththan Arul" by Sri Velayudham Karthikeyan.)
On the other hand, King Ravana, once he began to understand the vast powers that came with the boon granted by Lord Siva for his austerities, he became arrogant.
Born in the lineage of one of the most learned and wise sages of all time, Rishi Pulatsya, Ravana had all the bearings that could make him a legend. Not exactly a hero, which he could have been, he definitely had the knowledge to be one. But, his ego, his illusion of immortality, and the belief that he would never be punished for his sins made him fall in the battle of Ramayana. Certainly, an interesting character, where on one hand, he stopped any kind of animal sacrifice in his kingdom and was pious and ascetic, on the other hand, he had a long list of curses received from many celestial figures, including Nandi. That said, Ravana had his own share of sins that he committed, and before he could have realized his sins (maybe he would have never had), he met his inevitable death when Rama's arrow pierced his navel. (Source: https://www.speakingtree.in/allslides/rise-and-fall-of-ravana/49116)
Agathiyar in my Nadi reading revealed that I had misused my power and authority as a Nambudiri priest in my last birth, harmed the poor and helpless, and manipulated the Sastras. Hence, I had earned the wrath of the learned and elders. So I was sent to perform Brahmaharti as a remedy in this birth by Agathiyar in my first Nadi reading. In my daughter's reading, Agathiyar tells her that she had desired her parents of an earlier birth in Papanasam to continue as parents in a future birth. Hence, my wife and I had come together to bring her into this world and raise her up.
As for most of us, there could be three possible reasons for us to take birth and rebirth. One could come with a mission, as with Sankara. Or to fulfill others' wishes, as with Sri Rama. Or to assist other souls in living out their lives as with masters, gurus and saviours. Or to fulfill certain remaining desires. Or to settle past scores. Many are the probabilities, possibilities, and combinations in coming again. This is what I guess Ramalinga Adigal meant in saying that the dead shall arise.
Even the Gods and Goddesses, we are told, were never spared. Arrogance on their part brought them to earth to live a mortal life and conduct extreme austerities, yagna, rituals, and meditation before they were accepted back into their fold. Similarly, it is with the numerous other stories of devotees who were lifted to an elevated stage in life, akin to ma manithan, deivam, mahan, gnani, muni, Siddha, rishi, etc, but due to arrogance fell flat on their faces and fell to the ground, fell in disgrace and fell into oblivion. Hence, the reason Agathiyar warns me to tread with caution, knowing to differentiate the good, bad, and ugly and to sieve through the illusion that Maya envelops us in, lest I should fall into the traps and pits or mines laid before us.
Swami Chidvilasananda in "Inner Treasures," A Siddha Yoga Publication, 1995, wrote,
“The Indian scriptures teach that the earth is the place where you come to work out all your karma, the consequences of your actions, both good and bad. This is where you have the opportunity to learn the greatest lessons and ascend to the highest awareness. Even celestial beings want to take birth in this place. It is the work of the Saints to awaken people from samsara, the world of the wandering, from this chakra, this wheel, the cycle of birth and death.’’