When Lord Mahavishnu appeared before King Prthu and asked him what boon he craved for, Prthu said that he sought nothing, for pleasures of this world do not last. Moreover, to ask a boon related to life on this earth would be like walking into a trap, that would keep one imprisoned in samsara. Prthu told the Lord that as the father of all, the Lord knew what His children needed, said P.T. Seshadri, in a discourse.
But people lack the knowledge to understand that He is resident in all, and that to ask Him for something is wrong.
The Lord was pleased with Prthu’s reply and said, “You are a great bhakta. As long as you are king, protect your subjects. As long as you are on this earth, do your duties. He who follows the rules laid down by Me will always prosper.” The Lord then left for Sri Vaikuntha. Prthu ruled well and in accordance with dharma. But he yearned to go to Sri Vaikuntha. The only way for that to happen was if a man accumulated neither merits (punya) nor demerits (papa). If he remained in the palace, he would continue to give to those who came to him asking for help. And these acts of charity would count as punya, and would result in another birth. His acts of merit would give him a happy life in the next birth. But that was by no means the equal of liberation, for liberation meant exiting the cycle of births and deaths permanently.
But what constitutes punya and what is sinful? Kooratazhvan answered this question.
He said that pleasing the Lord was an act of punya and displeasing Him was an act of sin. Doing our duties makes God happy. Prthu listed four duties of a ruler. A ruler must punish the wicked, protect his subjects, provide for the livelihood of his subjects and ensure that his subjects did their duties.