Sattvam rajas tama iti gunaah prakritisambhavaah
Nibadhnanti mahaabaaho dehe dehinam avyayam
— Bhagvad Gita, Chapter 14, Verse 5
When purush (soul) combines with prakriti (creation), in the form of three gunas — satva, rajas and tama — a being takes birth in creation. Light, happiness and gyan are properties of satva, rajas pertains to desires, attachments and resultant actions, and tama is darkness, ignorance and sleep. At all times, all the three gunas are present in a human being, one dominating the other, depending on the desire and state of evolution of being.
Ordinary beings are ruled by tama, which is also the guna dominant in animals and other lower beings. When a being leaves the body with the dominance of tama guna, s/he gets the animal yonis and enters into the netherworlds, says the Bhagvad Gita. So the tama guna needs to be reduced and satva and rajas increased.
As the rajas guna increases, one is guided towards action (karma) driven by passion, material desires and attachment. Every action has an equal and opposite reaction, every bhog has a rog attached to it. So under this guna, a being indulges in pleasures of the senses and experiences the pains that come with it. There is no limit to desire and no matter what one gets, s/he is never satiated and keeps needing more. This ties the person to the downward spiral of birth and death — each birth being lower and more painful than the previous. Therefore, excess of the rajas guna is bad.
Abundance of the satva guna, makes us practice dhyan and sadhna and initiates cleansing through service and charity. Gyan and bliss follow. When a being leaves the body with dominance of satva, s/he takes birth in subtler dimensions and lokas, in the yonis of devas and rishis. All the three gunas pertain to physical creation. We can exit the painful cycle of birth and death to merge with the ultimate reality, i.e. God by rising above these gunas. When we go beyond the influence of tama, rajas and satva we become guna ateet — a state that can only be achieved through a guru’s help. When the guru transfers his/her gyan (shakti) to the disciple, the latter becomes guna ateet.
But gyan can only exist in a satvik environment. So, it is imperative for us to engage in charity and service, to change the negative karmas accumulated over so many births. Only then can we move towards gaining gyan and liberation from the cycle of birth and death.