I friend of mine sent an SMS, "Our hopes are like hair and nails. No matter how many time you cut it, they grow up again." Provided a person is not suffering from some kind of biological disease that don't allow these to grow at all.
What can justify such an analogy? Freedom movements. Our rising up from falls or failures. Seeing the light at the end of the tunnel. Night's being darkest just before the sunrise. Yes the last 2 sentences are merely poetic and not arguments, but no matter what they do testify that humans do not fail to be hopeful. Perhaps only this explains existence and constant invocation of such idioms.