Evolution giving us a consciousness that is biologically hardwired to search for meaning in an inherently meaningless universe is the flaw in our design that causes us to feel a deep, yet subtle, feeling of uncomfortableness or wrongness with our own existence. It's easy to ignore this feeling if everything in your life is going well, but it becomes more apparent when things stop going well, and once you finally notice it then it can be harder to ignore later, even after other things have returned to normal. Suicidality is as natural as breathing in my opinion.