It's a weird topic; I've felt for nearly half my life acutely vulnerable to depression, but not to suicidal thinking. Depression is death. It is unfeeling, depersonalized, hopeless, void. Maybe other people experience forms of depression that are active, somehow, a positive, present sort of pain, from which they want a respite. But mine, whenever it appears — which it hasn't seriously for quite some time, for which I'm quite grateful — it's always negative.
Suicide does make for a decisive corner case in philosophical arguments: if suicide can ever be justified, then it seems so, too, killing can be (sometimes!) mercy rather than murder. Such a position implicates the existence of some zero-point of happiness: that the utility function takes values even in the state of nonexistence, and that it may range below this value in the domain of the existing!
Camus seems to be asserting that philosophy must explain not just why some people commit suicide, but even stronger to justify why anyone should not. It's an ambitious question. I know for myself I don't feel a need for a justification: my fear of the void of death is enough. But a justification would be nice.