I would like to know what our scriptures say about prayopavesa. If deciding to end one’s life is wrong, why is it prescribed in our scriptures?
Praya is fasting till death.
Those, who have .not been killed by water, fire, hanging, adopting the life of a mendicant, fasting, poison, falling down, Praya (fasting till death), and self-inflicted strokes of a weapon all these ; [as well as] the drunk, and those interdicted by all persons are purified by the Chandradyana and two Taptakrichhas. (2-3)
So, it is method of killing oneself. Then why it is not considered sinful like suicide?
Simple answer is -- because Scriptures mention Praya to be a meritorious method and it's an austerity that leads the soul to higher stations after death unlike suicide which all scriptures describe as a great sin.
See the following verses:
Manu Smriti 6.29. These and other observances must a Brahmana who dwells in the forest diligently practise, and in order to attain complete (union with) the (supreme) Soul, (he must study) the various sacred texts contained in the Upanishads,
6.30. (As well as those rites and texts) which have been practised and studied by the sages (Rishis), and by Brahmana householders, in order to increase their knowledge (of Brahman), and their austerity, and in order to sanctify their bodies;
6.31. Or let him walk, fully determined and going straight on, in a north-easterly direction, subsisting on water and air, until his body sinks to rest.
6.32. A Brahmana, having got rid of his body by one of those modes practised by the great sages, is exalted in the world of Brahman, free from sorrow and fear.
As one can see, verses 31 and 32 clearly mention the method of Praya as a highly meritorious one and it's also a part of an austerity that leads the soul to higher spiritual realms after death.Apparently, only a Brahmin is allowed to end his life in this manner too.
Suicide, on the other hand, is clearly condemned in the same scriptures which praise Praya.
Manu Smriti 5.89. Libations of water shall not be offered to those who (neglect the prescribed rites and may be said to) have been born in vain, to those born in consequence of an illegal mixture of the castes, to those who are ascetics (of heretical sects), and to those who have committed suicide,
So, the reason why one method is allowed while the other is not is simply because scriptures describe one as a meritorious act and the other as a sinful act.
If scriptures did not tell us that committing suicide is a sinful act then what was the way for us to know it? What is a sin and what is not we can't judge by senses and our limited intelligence. Such instructions must always come from a higher plane of existence which are the Rishi's wisdom and the scriptures. Because all Karmas have consequences on the visible world as well as in the world whose existence we are not directly aware of. For example, heaven, hell, deities, Punya, Papa, afterlife, rebirth etc. we don't experience these like we feel heat and cold etc. So all instructions pertaining to them must come from a higher authority which consider the scriptures are.
Without the scriptures, we neither know the consequences of suicide nor those of fasting till death. We know only because scriptures tell us that one is sinful while the other is highly rewarding.