Because it's irreversible, and people contemplating suicide are often being impulsive or illogical, yet feel as though they are being completely logical. Maybe

Because it's irreversible, and people contemplating suicide are often being impulsive or illogical, yet feel as though they are being completely logical. Maybe they're chronically sleep-deprived, or having a vitamin deficiency or drug-interaction. The problem is that they will think they are making a decision of their own free will. This is why things that affect your very thinking are so insidious. Think of sleep deprivation or hypoxia. Ever watched a video of someone trying to draw simple shapes while not getting enough oxygen? The crucial point is that they think they are okay. When I was feeling suicidal it seemed like the only logical solution to a set of unsatisfiable constraints. However, none of these were particularly hard constraints. As a relatively privileged person I was ashamed to reach out for help, because I felt I was letting everybody down. Don't be ashamed. It can do weird things like convincing you that it's better to kill yourself than inconvenience your self and others by e.g. quitting a job, or failing some courses. To make a coding analogy -- sometimes your client / management etc. tells you they want X and Y and Z. The set of solutions to X and Y and Z is empty. However, if the constraints were relaxed, X and Y is solvable. Y and Z is also solvable. Maybe they didn't really want X anyway. This closing down of options seems to be a common pattern, however it's often false. Very few people are actively being tortured to the point where death is the only logical escape. Typically, there's more of a spiral inward where more and more options disappear until only one seems to be left. Typically there are so many steps along this path that removing a few constraints or realizing that they're not really there can make all the difference.