Sattva means my existence. At the present moment my existence is not śuddha, purified. Just like if your health is contaminated, then you get some disease. So for the soul, the living entity, this disease, janma-mṛtyu-jarā-vyādhi-duḥkha-doṣānudarśanam (BG 13.9), this is a kind of disease.
Now, it is a fact because the soul is eternal. Nityaḥ śāśvato 'yaṁ na hanyate hanyamāne śarīre (BG 2.20). The soul is eternal. It does not die. It does not annihilate after destruction of the body, but there is change of body, mṛtyu. Janma-mṛtyu means change of body. Tathā dehāntara-prāptiḥ (BG 2.13). So people should be intelligent to know, "Why I shall undergo this tribulation of repetition of birth and death?" But they do not know it. There is life without birth and death. Tyaktvā dehaṁ punar janma naiti (BG 4.9). After giving up this body, no more taking birth again with this material body. There is a life like that. We get this information. Why should we not fulfill this mission of life in this human form of life? Why unnecessarily desire so many sense gratification? This is called tapasya. If one life we have enjoyed the sense gratification.... Sense gratification, āhāra-nidrā-bhaya-maithunaṁ ca. Eating, sleeping, sex life and protection from fearfulness. Now this has been done in so many lives. Why not in this life make a perfect process so that no more death, no more birth, no more disease, no more old age?