If we choose our circumstances, then some people must have chosen a life of suffering, perhaps to learn something valuable from it?
However, if suicide is never "part of the plan", why would someone choose a life filled with more suffering than they are capable of bearing?
There are some people who experience untreatable biological depression so severe and pervasive that there is simply no way they can possibly function; they are incapable of maintaining normal interactions with other people, incapable of supporting themselves, and are basically frozen or half-alive. For these people, suicide seems inevitable --- the natural response of a biological organism to a constant, absolutely overwhelming, agonizing stimulus; under enough torture, ANYONE will eventually pray for death.
(I am not describing myself, despite my earlier pessimistic post. From my own experiences with depression, I simply can't imagine the horrible pain that some of those hardest-hit must be going through; it makes me sick just to think about.)