It's simply a decision someone can make. In the end, the only person you're truly responsible for in life is yourself. Friends (= people you usually met simply because you were in the same classes or lived close to) don't matter then, neither does family because yet again, they are coincidental. You might have grown attached to them and they might have grown attached to you, but truly every one of us has always been and will always be alone. For some that realization alone may be reason enough, others use it for justification of their own needs, but regardless it's always up to the individual.
It would be nice to live in a world where nobody ever wished to die but unfortunately this is not it.