Testicles are very sensitive. Most guys would agree that getting "hit in the balls" is a painful experience. One of the reasons they are so sensitive to pain is because of the mass amount of nerve endings enclosed within the testicles.
Since testicles are responsible for the production of sperm (a very important function) the body wants to make sure the individual protects the area. Humans naturally try to avoid pain and instinctively protect their reproductive organs if they expect extreme discomfort.
Another reason direct force to the genitals is extremely painful is because the nerves are connected to the abdomen. Often, when a guy gets hit in the testicles, he feels the majority of the pain in his stomach. This is because the pain travels up the inguinal canal, which is the same pathway the testicles took when they descended into the scrotum.
