“What doesn’t kill us makes us stronger.”

What do these two quotes mean and how do they relate to Humanistic and Existential Psychology?

Philosopher Friedrich Nietzsche wrote:

“What doesn’t kill us makes us stronger.”

Philosopher Soren Kierkegaard wrote:

“Life must be understood backwards; but… it must be lived forward.”

Provide examples outside of your text and post references with your response to the question.