Americans have always had a soft spot for the underdog. Our natural compassion leads us to cheer for those who face great adversity and yet are still willing to put everything on the line, even when they know their chances of success are low.
This may be partly due to our own historical underdog status as a nation. Few battles began more lopsided than the Revolutionary War, where a band of determined farmers with pitchforks and shotguns took on the most powerful military in the world.