Healthcare providers and medical experts aren’t sure what causes varicoceles to develop. Some think that a faulty “switch” (valve) inside certain veins in the spermatic cord may cause them. The spermatic cord is a band of tissues that holds your testicles in place. These veins return blood from your testicles to your body. The valve acts as a sort of “on/off” switch. It has two flap-like structures that help blood move back toward your heart. When a valve doesn’t work the way it should, blood may build up inside the veins in your testicles. Over time, the veins swell.