Varicocele is common on left testis because -
Variocele formation has been attributed to one of the three primary factors :


Increased venous pressure in left renal vein
Collateral venous anastomoses
Absent or incompetent valves in the internal spermatic vein.
Left testicular vein drains into IVC which has high pressure
Left tesicular vein drains into left renal vein which has high pressure
Left testis is lower situated
Compression of testicular vein by rectum