Nirāmisa sukha is a “general sense of well-being” and NOT a particular vedanā (like when eating a tasty meal).
It comes gradually as the mind is cleansed of lobha, dosa, moha.
There is the following sutta where a former king/aristocrat who became a bhikkhu expressed it as “Oh, what bliss! Oh, what bliss!”
– The other bhikkhus thought he was thinking about the enjoyments that he had given up. But he explained that it was exactly the opposite.
“Bhaddiya Sutta (Ud 2.1)”
– We don’t realize the burden that we carry when we are enamored with worldly possessions.
– When you start seeing the “anicca nature” of those possessions, you gain nirāmisa sukha.
– It just happens over time.
Do things happen without causes?
– If there is no progress in understanding, it will not improve.
P.S. Unless one becomes at least a Sotapanna Anugami, any progress CAN even reverse. If we die before that, we don’t know where we will be born next. Even if it is in the human realm, one could be born into a “wrong family”.