If I understood correctly, during a meditation session, thanks to the mental calmness and to vipassana, an arhat can reach nirvana, so that he transcends the human life and ceases completely to suffer (including the physical pain).
But if the arhat goes out of this state, he will come back to the normal world and will start having physical pains again (even if he will have no more sufferings due to mental illusions).
Therefore, I do not understand why arhats do not remain perpetually in the state of total nirvana, until the body dies of itself and they reach parinirvana.
I must be making a confusion somewhere. Please help me.
From what I can recall, one of the first thoughts to occur to the Buddha was that his realisation goes so against the stream of the world that people may not understand. He then inclined towards not teaching, and simply sitting in meditation. However, Brahma Sahampati noticed this and went to the Buddha to ask him to teach. Then the Buddha properly surveyed the world and found that there were some people willing to learn. So, out of compassion, he started teaching.
Since then, it seems like every Arahant after him followed his example. So I would say that the main reason for Arahants to not simply dwell in meditation and die is that they wish to dedicate the remainder of their lives to helping other living beings.
A certain amount of compassion is a prerequisite for awakening, and once a being is awakened, that same compassion motivates them to help others. Because the Arahant is free from clinging, they don’t suffer mentally or emotionally on account of the hardships they may face in the course of doing so.
There were a few Arahants who got ill, where the illness had progressed to the point where other monks were taking care of them. It seems that they decided that living was more burdensome for themselves and the monks who were taking care of them, and for that reason committed suicide. Incidentally, the only kind of suicide not considered blameworthy in Buddhism is that by an Arahant. The suicide of a normal person is considered blameworthy because, in the words of the canon, one gives up a body in pursuit of another body (i.e. you die seeking birth somewhere else).
Appropriate attention continues to involve making choices, though they are now strengthened and skilled in doing so. ‘Strengthened’ means views and perceptions (including emotional responses) have been straightened out:
“when the arahant after full awakening
engages in right mindfulness, it’s with a sense of being disjoined from body,
feelings, mind, and mental qualities. At the same time, he/she continues to
engage in appropriate attention. Although the purpose now is different from
that of an unawakened person, there is a purpose nonetheless.”
The duality of nibbana and samsara continue to be experienced concurrently but skills have been developed where every situation is overcome. This involves skills in both insight (seeing) and tranquillity:
“There are fermentations to be abandoned by seeing, those to be abandoned by restraining, those to be abandoned by using, those to be abandoned by tolerating, those to be abandoned by avoiding, those to be abandoned by dispelling, and those to be abandoned by developing.”