Just a few thoughts here, no grand treatise. But you should read up about this, and choose it only when you are highly knowledgeable and able to make a good choice!
There is no fixed answer or "right" answer to this question, but it is worth asking, I think - ahead of time.
The quality of our living of life is strictly based on the quality of our experience of life. That experience of life is based on the actual circumstances, to some extent, but much, much more (almost all) on what the meaning is that we add to it.
Surely physical pain is a key factor, probably the only one that causes the balance legitimately to go to "the other side", the negative and unchangeable "bad" experience of life. Though I do note that pain can actually be tolerated and life can still be happy. See Pain, note the experience of the monks (who have trained their perspective).
Notice that people who we think are pretty well living a miserable life even near the end want to hold onto it. I think there is something to be had out of just "feeling" and experiencing being alive, no matter which level.
However, if I am in too much pain and I am terminal with no hope of managing, I would like to choose to take the Grand Exit - where I acknowledge that it has been a real trip, a real miracle to having had a life, and that I appreciate it all... and, now, good bye, farewell, arrivederci, adios....
Notice one who is not presently in his/her right mind for making good decisions should not be making the decision. It must be made by that person when in a rational state or by another fully informed person who is in a rational state (that is not dogmatic and fixed or "moralistic" but ethical).
