Why are a guy's balls, testicles, a symbol of strength, power, aggressiveness, bravery, maturity, etc.?

Anonymous
and Responsibility.

Most people never really bother to explain why, basically whenever there is a situation in which a guy is acting immature, or being passive or scared, nervous in a situation of any kind, people will always tell him to grow a pair, grow some balls, etc.

Seriously, so why are a guy's ball's, testicle's, a symbol of strength, power, aggressiveness, bravery, maturity, responsibility, etc.
Why are a guy's balls, testicles, a symbol of strength, power, aggressiveness, bravery, maturity, etc.?
12 Opinion