It's just because others don't really understand as they are not living our lives. People live under this delusion that things will always improve, they don't want to face the fact that life is not worth living for many. If someone is close to dying from cancer then I guess that others might see the desire to die as being rational as that person will die soon anyway, so dying sooner would prevent more pain. Terminal illness is objectively horrible to people, people have heard about how it affects those suffering with it and they wouldn't want to end up in that situation.