purushah prakriti-stho hi
bhunkte prakriti-jan gunan
karanam guna-sango ’sya
sad-asad-yoni-janmasu

"The living entity in material nature thus follows the ways of life, enjoying the three modes of nature. This is due to his association with that material nature. Thus he meets with good and evil among various species."

prakriti-stho = in this body (made from the Five elements)purushah = Atman, bhunkte = experiencesprakriti-jan gunan = the Tri-Gunas ever associated with body. This is what Nammalwar also implied in his reply. In the second line of the sloka it is asked what is the reason for the Atman to continue in infinite birth-cycles to experience this worldly happiness and sorrows? karanam guna-sango ’sya = reason being the Atman is associated with the Tri-Gunas. Sad-asad-yoni = taking birth in higher (human or Deva beings) and lower (insects and other lower) forms of life. Here it might puzzle us. It is like asking for directions of Mr.A's house. The reply given is it is next to Mr.B's house. Where is Mr.B's house? It is next to Mr.A's house! This will lead us to nowhere. Like thios in the first part, the Lord says, the Atman experiences Tr-Gunas and so gets repeated birth cycles. Why the birth-cycle? It is because Atman experiences the Tri-Gunas! Is Atman residing in a body to experience happiness and sorrows? Or, is Atman taking re-births because of enjoying the Tri_guna resulting in birth cycles? This is like chicken first or egg first? Here a question may arise. The Atman is so great. Why not it experience its pleasures itself? Why get associated with a body and get involved in the Tri-Guna experiences? Atman needs a body to experience and while in a body if it acts according to shastras, it gets Punya; but, if it acts otherwise, it gets Papa. According to the acquired Punya or Papa, the Atman gets re-births.    The soul in the body has to feel the happiness and miseries as they are inevitable in this World. Now we can see the equation correctly. Atman desires in pleasures and so Atman resides in a body and experiences. Because of this association with the body, Atman does so many activities, which make to acquire more and more karma. Thus cause and effect are the same. That is, Atman takes a body as residence to spend karma; while being resident, more and more activities are performed, which make Atman to acquire more and more karma. It is a cause-effect - cause cycle. Karma causes to reside in a body and enjoy [or suffer] effects. Effects cause more actions and so more karma accumulate as effects. Such acquired karma causes again residing in a body, and so on. Purusha =Atman, prakruti-stha = residing in body, gunan = [satva, rajo and tamo] qualities, prakruti-jan = always associated with body. These three satva-rajas- tamas qualities are always associated with the body. An action is propelled by these three qualities. That is why pleasures and pains. Thus Atman housed in a body, experiences happiness and sorrow, a result of the actions is driven by satva-rajo-tamo qualities of the body. Sad-asad-yoni-janmasu = [thus, Atman] takes a variety of births in many bodies [like Devas, human, animal, bird,etc.], karanam = reason being, guna-sangha = [attachment to] the group of [satva-rajo-tamo] qualities. That is, the attraction or attachment to the pleasures and miseries, generated by the actions caused by these qualities. This attachment causes more and more births. This is what Nammalwar gave as the reply to Madurakavi Alwar's question. Atman, again and again, takes birth in bodies. That is, Atman, experiencing the happiness/sorrow as results of the actions of the body and remaining always in a body, in every birth. Therefore, this sloka is closely connected with Nammalwar.
                                                                                                         (continued)