Barring a terminal diagnosis, or a sever physical deformity or disability, my thought is the exhaustion of all coping strategies. Has a significant effort to improve the self/environment been attempted? It's such a personal thing, and such a personal choice, I think there are many different reasons someone might decide to end it, and it might not make sense to others, but would it have to? From my perspective, it seems like more of a way to gain freedom and control for the self, and a lot of it stems from a conflict with personal desires clashing with social expectations.
For the past two years I've been trying to kill my old self, and shed the mind-policing attitude that kept me doing what I thought would make others happy, instead of what would make me content and satisfied. I don't know what I'm saying, but we'll all be dead eventually anyway. It just sucks to live a lifetime in your head, and not pursue any dopamine-producing activities.