There are two big mysteries in Hindu mythology. One, why does Kaikai ask Ram to be exiled only for 14 years? Two, why does Duryodhan demand that the Pandavas be exiled only for 13 years? Why did Kaikai not ask for Ram’s permanent exile? Or why did Duryodhan not ask for the permanent exile of the Pandavas? Why was the exile in the Ramayana and Mahabharata for a fixed tenure and not permanent? The answer may lie in the idea that, in Indian thought, nothing can be permanent. The only thing that has permanence is the soul or the atma. Everything else is transitory. 

A clue to this are the stories from the Puranas. In the Puranas, asuras keep invoking Brahma. They then ask him for immortality. Brahma refuses to give this to them. So, the asuras try to outwit death itself. They ask boons that make it impossible for anyone to kill them. For example, Andhakasur can only be killed if he falls in love with his own mother. Mahisha demands that he should not be killed by any male creature on Earth. Tadaka asks that he cannot be killed by a single adult, not considering babies as a threat. Ravana asks that he cannot be killed by any god or beast, not thinking humans will be a threat. If one pays attention, all these boons leave loopholes. These loopholes enable the devas to kill the asuras.