Fear

Fear reminds us that we are vulnerable; that we are weak.
Feeling weakness, pain and getting hurt; reminds us that we are human.

Then comes the challenge. Do you let these things stop you, and quit? Do you back down when things get rough? Do you stop when you can't breathe or when your body is damaged? Can you only perform when you are at 100%? The challenge is to overcome. And this reveals humanity's strength: the human will. To survive. To succeed despite weakness. To overcome weakness. We don't succeed because we are godly. We succeed because we are human.

Happy is the person who isn't distracted by discomfort, that (s)he can appreciate even life's simplest pleasures.