What is the yardstick of value you're judging life against?That is up to each individual person. In my opinion, if I'm miserable for the majority of my time and I'm not enjoying being alive at all anymore, then in my opinion my life has stopped being worth living and I'd personally rather end it on my own terms than continue living just for the sake of it. I personally don't believe I get anything from living just to stay living and would rather end it sooner than I otherwise naturally would.
For other people, its all about surviving and things would have to be super horrible for them to consider taking their own lives. Its a personal thing and I think everybody should be allowed to choose on their own terms based on their own beliefs and opinions.
Of course, its not that simple in real life because what I value my life at isn't necessarily what I should be valuing it at. For example, I may feel its time to end it based on my criteria above, but if, say, I had children who relied on me, then that needs to be factored in too and life may well be worth it for their sake if not my own.
But at the end of the day, its my opinion that if (my life's value to me) - (my life's value to others) < 0 then people should accept that its my choice. In a lot of suicide stories I've heard people say stuff like "how could (s)he have done this to their loved ones" or "suicide is a selfish act", but this ignores the pain the person was going through. IMHO if my pain being alive is greater than the pain my loved ones if I'm dead, then it would be selfish of my loved ones to expect me to continue to endure my pain just so they don't have to deal with theirs.
(yes, yes, in real real life its much more complex since often my perceived value is based on temporary circumstances, mental illness etc and not a true reflection of my self-worth or willingness to keep on living)