I think the most common/legit reason that most everyone agrees on is when you have a chronic illness or debilitating injury that will never get better but will instead decrease your quality of life as you go on with life.
Second one would be older persons who have a reduced quality of life and are losing loved ones due to age and age related complications.
I think that's a very subjective question, what's legit? Some say those with physical ailments (that's why I'm looking to CTB, because the condition I used pain medicine to control, I now can't get pain medicine for because of the addicts. The only relief I have left is CTB.) But who's to say that someone's mental anguish isn't just as excruciating? Even if it's not an actual condition. Maybe they're just tired of living. I don't think it's really fair to look at anyone through this scope. Anyone who CTB obviously had a reason that was valid to them, and that's the only one that matters.
I'm somewhat biased since my reason for wanting to CTB is multiple chronic illnesses that leave me with almost no quality of life.
And to be honest, sometimes I see posts here that make me cringe. "Like seriously, that's why you want to CTB?" But I've come to accept that we all have different limits. We can never know what life is like for other people. We can never truly understand what another person is experiencing. And therefore we have no right to judge another person's reason for wanting to CTB.
Chronic illness or debilitating injury by far since it's not "all in your head" or a perceptual error. It affects every minute your life you are awake and thing with health issues is that they don't even have to be big, but they build up. Imagine a situtation where you would get a small papercut on your finger and it would never heal, it's not a huge injury, but that nagging pain will drain you.
Suffering is not a contest, but everybody who's healthy should really look into their life and think are their situation only temporary or not. And is it really worth killing yourself over.