The exact cause of varicocele is unknown. It's generally believed that a faulty valve in certain veins within the spermatic cord, which is a band of tissue holding the testicles in place, may be responsible. These veins return blood from the testicles to the body. The valve acts like a one-way gate, with two flap-like structures helping blood flow back towards the heart. When a valve malfunctions, blood pools in the veins of the testicles, leading to swelling over time.

Varicocele is found in 15-20% of men in the general population, even those who have fathered children. In men seeking treatment for infertility, the prevalence rises to 30-40%. In cases of secondary infertility (men who have had children before but are now seeking treatment for infertility), this rate can reach up to 60%.

Varicocele occurs on the left side in 90% of cases, bilaterally (on both sides) in 8-9%, and on the right side alone in only 1-2% of cases. The higher incidence on the left side is attributed to several anatomical factors:

* The left testicle is positioned slightly lower than the right.
* The vein on the left side is longer than the one on the right.
* The anatomical relationship of the left testicular vein with neighboring abdominal organs.
* The anatomical structure of the drainage pattern of the left testicular vein.